Boggin
For several months I've been experiencing frustratingly long response times on the net.  I use Firefox and Bing.  Typically after a reboot, I'll get very good response times (a second or so) with the first 6 - 12 urls.  Then response times can be measured in minutes.  Quite frequently the request just times-out.  I tried to swap between Userids, but this made no difference.  I tried IE and Google.  The same.  I have no choice but to reboot.  Just occasionally and very confusingly, I can use my system with good response time for up to 30 minutes before the problem re-appears.
I've checked line speeds (BT with BTInternet).  Downloads between 5-6Mbps, uploads between 0.5-0.6Mbps.  Good for our rural area.  I've tried various Malware products, singly and in groups.  These include my own Anti-virus Bitdefender, Malwarebytes, SAS and Spybot. With no obvious improvement using this technique, I checked the Web for "How to identify and resolve bottlenecks in XP".  That's when I cam across Tweaking.com.
Hope that helps
Peter