Internet connection problem (XP)

I am in the midst of a pretty bizarre and frustrating issue.

Essentially there is a problem with the way my system is interpreting and reacting to my incoming cable internet signal.

Here are the basic bullets of what's going on:

- the system runs fine when booting without being connected to the internet
- upon connection, the system begins to slow
- the connection functions properly, although rather slowly, for a few minutes
- after a few minutes HTTP requests are no longer able to be processed
- if connection to AIM or other instant message client was established during that window of proper functioning, it stays connected after web browsing begins to fail
- the modem shows no problems, as does the networking area of the task manager
- at this point the entire system is in bad shape, with CPU usage rates fluctuating untill it gets to the point of constant 100% usage

This is a desktop running XP with SP2. Scans have come up negative for viruses and spyware.

Anyone have experience with a similar situation or have some general insight to provide? I really need to solve this as this is my main computer. Any help is appreciated.

 

Only to ask if you followed ALL the advice in the malware sticky:

http://forums.majorgeeks.com/showthread.php?t=35407