There is a category of software pests that, in my opinion, are worse than viruses. Known variously as adware, spyware, or malware (some would argue a difference between them), these are generally professionally written by a software development company specifically to burrow into your computer and send information about you back to the distributor of the spyware.
They are often harder to get rid of than a virus. Some have several pieces, the components are deceptively named, they self-repair if you delete just a part of them, and often will change their own file or registry entry characteristics, all in an attempt to prevent removal.
They are imbedded in freeware and shareware programs, which is the main way they are promulgated.
They will interfere with performance by using your network connection without your knowledge, preventing other programs from using the Internet at those times.
Most of these programs are perfectly legal, believe it or not.
If you browse the Internet, you probably have some of this stuff right now. You can download free programs like AdAware or SpyBot to see. Be sure to update them, just like virus software, every time you run them.
