Basically because most people are constantly adding new programs, game demos etc and then removing them.
These programs always leave something behind, usually in the form of a registry entry.
As a result the registry gets bigger and bigger and the PC gets slower and slower.
The only real solution is to format your hard drive and start afresh.
If you do this, remember to export your Favourites folder, Your My Documents folder, your Outlook address book and Inbox.
Also, check the C:\Windows\temp directory and delete any files that are there.