Now let me try to address slow, sluggish, and non-responsive computers.
This seems to be the number one issue I come across. The PC just doesn't keep up like it used to.
I am sure others have there own methods of addressing this problem. Here is what I do.
The absolute worst is a completely non-responsive computer. If you boot into Windows and cannot do much because of how long it takes to respond, you may try going into safe mode. To do this, simply turn the computer off or restart it and start pressing F8. I generally press F8 at 1 second intervals just to make sure the computer is reading the command. I will cover what to do in safe mode a bit further down.
If safe mode isn't helping, your only choice (before a reinstall) may be using a boot disk with programs already loaded on it to help you access your system.
For a decent free boot CD, go to ultimatebootcd.com and download the UBCD. you will need a computer that can burn a CD. You can then use this CD to boot your system to a user interface that has some helpful programs. Definately run the Antivirus when you do this.
You can also create your own custom boot CD. Keep in mind, this is no simple task that you can learn in a few minutes. Your best bet is to create the disk when your computer is working fine instead of an emergency situation. This is really just FYI about a helpful tool that you should have in your arsenal. Check out BartPE/PE builder. There is a whole slew of plug-ins available for you to create your own custom boot disk. Again, this is no 5 minute task.
Depending on the slowness of the computer I will then go into the regular Windows installation and view the task manager. Click on the performance tab and it will show all of the running processes. At the bottom of the list is the "System idle process" which is the percentage of your processing power currently not being used by other processes. For cleaning your computer, I suggest you first make sure you aren't running other programs. On a very slow computer, I go into the processes tab first to see what is eating all the power. If you have a single or a few processes eating up the CPU availability, you will want to write down the process name and google it to see exactly what the process is and does. Don't automatically remove anything unless you know exactly what it is first. Then do some research on the best way to remove or stop the process. Again, I only do this first if the computer will not respond in a decent amount of time. If your computer is just running slower than normal you can go to the next step.
Start by running a good updated Anti-virus scan. There are plenty of free ones available. You can even run Trend Micro's Housecall for free over the internet.
Next up is Anti-Malware utilities. These are designed to get rid of Spyware and Ad-ware Every program you find will have it's own weaknesses. I suggest you use at least 2 on-demand scanners such as Ad-Aware and Spybot - these are just my personal choices, there are many more available.
After the AV and Malware tools have done there job, I like to use Hijackthis. It is really more of an advanced program and you should know exactly what you are doing before you remove anything with this program. A novice could still use the program by doing the following. Dwonload the program and run it. Choose the "Do a system scan and save a log" option. A notepad window should pop up with the log. Copy everything from the log file and paste it into the text box on this site and click analyze. You will then get a list of registry and file settings with alerts to possibly harmful entries. If it is marked as bad or questionable, read a little more about it by using google. It could be completely harmless or totally dangerous which you will then need to remove.
After removing all the nasties from the computer I suggest you then go into control panel and open "Add/Remove programs". See what you have in there. Windows lists how often you use the programs and the dates last accessed. Get rid of anything you don't use after making sure it isn't an essential program for any of your other programs or hardware. Keep notes on what you remove so if anything goes wrong you will know exactly what happened. It may prompt you to reboot to finish uninstalling - go ahead and do so each time it prompts you.
Now, lets see what you have running at startup. The number of processes that insert themselves into your startup folder can be amazing. Trust me, you usually don't need all of them to run at startup. On-demand works just fine with most programs. For Windows XP, click "Start" then "Run" then type msconfig and hit enter. Click the "Startup" tab to see a list of programs that automatically run at startup. Some of them are needed, others can cause your computer to take considerably longer to start. Go through the list and google the programs that you don't recognize. You can then uncheck the boxes for the ones that you don't need at startup. Write down which ones you disable in case it causes a problem. After you disable some of them click apply and OK. You will then be prompted for a reboot.
Now we need to get rid of your temp files and leftover registry entries. Some people like to do this manually. Personally, I like to use ccleaner. Just a couple of quick steps and it is all over - it also creates a backup which can be helpful. Download and run ccleaner for free. In the cleaner box there are 2 tabs that lists everything it will remove. You can check and uncheck these based on personal preference. Click the Analyze box to get a long list of everything and the "Run Cleaner' box to get rid of it all. Then click on the "Issues" box. This deals mainly with the registry. When you remove applications/programs from your computer sometimes they leave registry entries behind. Click the "Scan for Issues" box and then "Fix selected issues". There are quite a few other features available from ccleaner worth looking into if you have the time.
I then like to use regcleaner from Juoni Vuorio. This has a very simple interface but can cause major changes to your computer so read up before doing anything other than the following. Download and run the program. At the top, click on the tools tab and then move your mouse to "Registry Cleanup" and select "Do them All". It will then quickly scan your computer for various un-needed registry entries and present you with a list of all of them. At that point, at the top click on "select" and choose "all", then click remove selected at the bottom of the windows. Look at all that junk go bye bye.
This entire process should do the trick. You can finish everything with disk defragmentation. Click "Start", "All programs", "System Tools" and "Disk Defragmenter".
I hope this helps. I am not saying this is the only f**l proof way to clean up/speed up your computer, this is just what I personally use.