Internet options
delete files
delete cookies
(wait for each to finish, may take some time if you don't do it regularly)
restart
May speed you up
I second that.
You may also wound up with a lot of “addons”, spyware, etc.
Have you run any spyware checks lately.
This is very often the problem with slow computers.
For example:
http://spybot.com/en/index.html