The fact that it is an intermittent problem actually suggests hardware. And the piece of hardware that causes the problem can appear totally unrelated. I can name a few examples where an unrelated piece of hardware (or driver, for that matter) was the culprit.

I suggest you get a firewall. On a default Windows setup, there are a lot of security holes that are hard to lock down without a firewall. File sharing is one of them, and can be tricky to completely remove manually. If you want a free firewall, Zone Alarm is quite good.

Another thing to try: when this problem occurs, are you still able to ping the affected sites (if you don't know how, then go to Start and then Run, type in "command" and hit OK, type in "ping " and the site name (e.g. "ping www.google.com") and post the results)?