First, download adaware and spybot and run them both. www.lavasoft.com for adaware www.safer-networking.org for spybot. Allow spybot to create a backup of your files. Do full scans for both and immunize with spybot. When done go into add/remove programs and remove any items you no longer use. Next empty your recycle bin. Then, go to start, programs, accessories, system tools and pick disk cleanup. When it finishes analyzing choose all except "compress old files" (it takes forever) and then allow it to run. When finished go back to start, programs, accessories, system tools and pick disk defragmenter. You can run "analyze" if you want.. it will either tell you to defragment or not but you will most likely need to so you can save a bit of time by just defragmenting. Let that run and then reboot when done. This should speed things up considerably.
Hope this helps! :)