Any program that tells you that you have loads of viruses and you need to buy their software IS A SCAM.
Thousands of people pay out �29.99 or whatever for a pirce of useless software when they probably have not got any viruses at all.
Run Norton anti virus, and if it tells you that you have no viruses you are probably OK.
It may be worth installing am anti malware product like Adaware (free) to see if it can find and remove this program that is telling you to buy their software.
I beieve Spybot Search and Destroy can also do this (is also free) but I rarely use it.
While you should install and run only one anti virus product it is fine to run more than one anti malware / spyware product.