Why has my computer slowed to a crawl?

I downloaded netscape yesterday, since then my computer moves at a crawl. I’ve tried uninstall and system restore, and I’ve had no difference. When I go to task mgr. (WinXP), the slightest action sends the CPU (P4, 1.6gh) to 100% for quite a while. I have no clue what happened or what to do. My computer is almost useless at the moment, it took over 20 minutes to get to this site. I had to do it through my e-mail links, because when I try Int Exp., I get “this program is not responding.” please help.