Some of these programs that try to encourage you to buy their software and part with your money are not viruses but spyware or malware.
Some of the them can be VERY VERY nasty.
We have a laptop at home and someone in my family (son or daughter or her boyfriend) managed to download a program called Spyware Defender (and maybe other spyware as well).
This totally wrecked our computer. It stopped us getting to the internet, changed settings in the Windows Security Centre, replaced AVG anti virus with itself , and updated a Windows system file called "hosts" to direct us to only their web sites.
I spent ages trying to get rid of it, and each time it managed to reinstall itself.
I know a lot about computers, but I was not able to get rid of it, and in the end decided the safest bet was to reinstall Windows from scratch.
I did discover this piece of software that hunts for malware and spyware, suggest you download it, install it, THEN REBOOT. Then check for updates, then run it.
It offers a basic scan, and a deep scan. The deep scan takes a while but it is best to run it.
http://www.malwarebytes.org/