Was coming here to ask for some help with a similar kind of problem and was surprised to see this thread.
For about 3 weeks the net from my desk top has steadily become basically unuseable. I have xp pro and use firefox for most browsing and IE7 for a few others, mainly work. The desk top is linked by wireless PCI card to my router.
IE gives a message "cannot display web page", while Firfox gives "server not found" maybe 50% of the time, often when it does reach the web page it goes into a basic or disrupted layout or doesnt load some graphics. The pages which most often cause problems are things like yahoo and BBC. When things go well the content of the page loads but embeded adverts may fail and give the server not found message. Numerous reloading usually gets there in the end.
I've been concentrating on my wifi, and have had my ISP out to check their stuff (virgin cable 10mb) - although the reported speeds have improved as a result of these efforts the underlying problem remains. I've removed my mcafee and defender all to no avail. I've checked for malware and viruses.
I'd already checked the fans in my case before reading this thread and have since looked at the event log (though I don't really know what I am looking at) it doesnt seem to have anything nasty.
Judging by the others with vaguely similar issues here I wonder if the updates from Microsoft around that time may be a factor? Bushfiva's comments make sense but he is too cryptic for my knowledge level to be able to solve it?!!!
BTW I have found firefox as slow or slower than IE since version 3 too.
Any help? I have pretty much run out of options.
Thanks