Why are there people who create viruses and malware? There are a varity of reasons, and none of them are good.
What is virus software and how does it work.
Check your internet provider
Microsoft Security Essentialls
Most of the commercial anti-virus software comes with malware, spyware, and trojan protection. Normally this is enought to protect your system (if you keep them up to date). Occassionally, something bad will get though and your virus software won't be able to clean it up. This is when you can run standalone anti-spyware and malware removal software.
In the past I've used the following free software utilities to clean up computers (I downloaded the free version):
There are many others that may perform quite well for you. These just happen to be two that I know work.