persistant 'can't find server' and other messages when surfing

In addition to the very slow speeds I have already posted about I am now almost unable to surf the net at all. I receive one of the following messages almost each time I try to go to a new page.The first message is 'unable to find server'; for ex: it has said "unable to find the server at google" and google is my browser. Another message I receive is 'unable to load due to an internal server error'. A third is 'the server at address has timed out.' When I try again I get the same thing. I am also almost never any longer able to get into my email inbox at my isp. When I called the isp they had me reset the winsock thing, and now things seem worse. They claim its my computer, but I can get into my other non-isp email accounts, and my computer works fine when off the internet. Anyone know what is going on? I run malware anti-malwarebytes every day and ccleaner. I have windows xp prof, ser pack 2, and recently DOWNgraded from firefox3.5 to firefox 5.
Can you recommend a scanner to download perhaps?
Thanks for any help.
Sheena