the answer is you collect junk in the registry and left over bits of errors and files that windows isnt very good at cleaning up. its not complicated, but you should reinstall windows at least once a year. personally, i've been using Ubuntu, and it's similar to windows but it takes care of itself automaticaly, its free, it doesnt get viruses, and it has a lovely community of supportive users.
Link to Ubuntu (free alternative to windows): http://www.ubuntu.com/
If you would rather stay with windows, that is fine, and here is a link to CCLEANER, a popular program which easily (a few clicks) helps clean out your system. Use this from daily to weekly. And remember to always back up your files! CCleaner: http://www.ccleaner.com/
Other things you can do..
* Defrag weekly
* Clean out temporary fiiles.
* Install "Portable Applications" which is software that runs without having to be installed. This means there is no messy integration into the system, and no clean up necessary! Search google or better yet Wikipedia for portable applications.
* Learn how a PC is configured, built, and what components it contains. Expand your knoweledge of this magical box you use!
