First, I suggest running a virus checker and spyware checker. I use AdAware and Spybot for spyware detection. Both are free dowloads - do a www.google.com search.

If those things don't find anything unusual and it's still running slow you might try going to start>run and type in sfc /scannow (there's a space after sfc). That should fix any damaged Windows files, although you will be prompted to insert your original operating system CD if it finds an error.

 

The fist two links below may be helpful.