If your machine is running slowly you might want to Defrag your machine. I prefer a program called IObit to do this as although the built in Windows is ok on xp, Vista doesnt actually show you whats it doing or its progress. Also another benefit of this program is that it tends to arrange files onto your harddrive in a much more logical and meaningful way than Windows does. Which basically means that your OS can retrive the information that it needs too from the harddrive quicker thus speeding up your machine!
http://www.iobit.com/
I picked this up yesterday on your suggestion, I really liked it, it does seem to make more logical decisions than the windows standard.
It was pretty quick too even on an in depth pass rather than the quick option.