If you get the impression that your computer gets slower the more you use it you are probably correct.
There are many temporary system files and registry files that are written to and deleted from all the time. Some programs leave little artifacts all through your system when you uninstall them.
There are some tools built into windows that help to some extent however here is a link to a highly recommended (Free) tool that preforms great

I will spare you all the hype and advertisement. Here is the link to the page on CNET.com where you can read about it and download it for free.

http://download.cnet.com/Advanced-SystemCare-Free/3000-2086_4-10407614.html
Happy downloading. Good luck and please don’t forget to tell us about your experience with the program.