theres also a good chance your pc maybe just be a bit too slow for L4D, what graphics card are you running with that p4?
Try playing L4D single player before you waste your time with comcast. If it still lags you know it's the pc.
Daily Driver- i5 2550K @ 4.8, 8gb 1600 Kingston Hyper X Enermax Revolution 1050Watt, Gigabyte UD4, GTX 590, 128GB Crucial M4, 128 OCZ Vertex, 1tb 7200.12, X-FI ExtremeMusic. Hyper 212+.
Test Bench- Xeon W3570, Gigabyte EX58-UD5, HD5850. Lepa G900 PSU, 40gb kingston SSD Custom Bench
Media PC- ASUS E35M1-I DELUXE, 4gb Kingston, 64GB Crucial C300, Antec ISK 310-150. BD ROM
Server- Phenom II x4 840, 4gb Corsair, Cooler master 360. 2x2TB Seagate. WHS 2011