May we suggest... AdAware
As you browse the world wide web, some sites that you visit (even some pop-up advertisements) will download files to your computer, generally for marketing purposes.
By themselves, most of these are relatively harmless. As they stack up, however, they could cause performance problems for your computer.
Lavasoft has written a handy little utility called AdAware which, when installed and run on a regular basis, will scan your hard drive for any of these hidden files and delete them for you.
AdAware comes in three levels; the "plus" and "professional" packages are available for a fee.
The basic version, available for free, is quite sufficient for most users' needs.