The computer specs may help, but at a guess!
to much rubbish installed and running at the same time! Bet the toolbar area on Internet explorer looks crowded!!
quick fix, uninstall all software you don't use from add remove programs, and stop any startup processes you don't need by typing msconfig into the run box on the start menu and disabling stuff in the startup you don't need (use google if your not sure what they are, only do one or two at a time and restart to make sure no problems)
best fix, total reinstall and be more careful about what rubbish you install afterwards