Mysteriously slow internet on one computer

Hello,
I have a cable connection. I have two computers connected by direct cable cable and another by wireless connection. Out of these, one of the former suddenly started to have slow internet. Few points that may be significant:
1) The two other computers (cable and wireless) are completley fine.
2) THe computer in question has run a lot of spyware/virus removal programs and removed a lot (and hopefully only) malicious files.
3) It is extremely slow to load a new website. Startup of firefox/aim is also very slow. Curiously though, once on the website, browsing within it is fast as it should be.
Thank you for your help.

ps a more minor annoyance but on my firefox tools->options->network->settings, the default setting is at "manual proxy configuration," in which I have manually change it to 'direct connection to internet" everytime I open firefox. I can never set this setting to stay in effect.