How can i clean this pc?

I have the Dell Xps 630 and want to clean it with compressed air. Do i unplug my pc…? Also where are the fans located on this pc =)

After 2.5 years, my Dell XPS has become too slow. I use all of the spyware (Ewido, AdAware, and Spybot) and virus apps. (E Trust Internet Security Sutie) and have never had a virus. I use CleanUp! and WindowWasher and Registy Mechanic to no avail. I think the registry has simply gotten too huge. It is time to do a clean install of XP Pro.

i just bought a dell xps m1330 it runs on vista after using it for 4 days with internet it doesn’t start up.. it say’s Windows failed to start. A recent hardware or software change might be the cause.
ive been givin options i chose Click "repair your computer"
after that all i see is a black screen.. how do i fix this ? i also insertes the OS nothing happened.