If you buy a PC nowadays, it’ll come loaded with loads of crap you may or may not want: trial versions of software with nag screens after a period of time, the system builder’s own software, stuff that shows you ads, google desktop and toolbar etc etc etc.

PC World has a good article on this here.

To get rid of it there are a few clever freeware options:

PC Decrappifier is a good one for this

Dustbuster gets rid of temporary files and directories

There are more tools out there, mainly paid ones. Have a look here.