This generally happens with Windows machines over time as they bloat up with caches, spyware, programs running at bootup..
The best solution I could find to run a clean-up of this was CCleaner, it cleared out my IE cache and other temporary files while giving me the option to pick and choose what programs open at startup.
Here's a link to CCleaner; http://www.download.com/CCleaner/3000-2144_4-10547048.html?tag=lst-0-1
recently my newly bought laptop has been going at snail pace. It takes about more than 2 minutes to fully load where before it took only half a minute. I recently got told that i had several trojans but the guys at CastleCops help me to get rid of them.