What shall I do to cure my computer from a virus which keeps on popping out windows?

My computer which is connected to the internet keeps on popping out dialogue boxes or windows saying that I have been infected with NetWorm.-i.Virus@fp or W32.mozart virus or that I need to download anti-spyware. Specifically a security warning says the name of the virus, the damage level, the description, the advice and the protection which is to download certified antivirus software by clicking the baloon. Upon clicking the baloon, selected antivirus programs are displayed which says free download but these are actually for sale because they will only scan your computer and show the files infected and advise you to buy their anti-virus softwares. I suspect that this in itself is a virus. What shall I do to stop this. I have already downloaded comodo firewall which is absolutely for free and pc ools anti-virus but the diague boxes and windows keep on coming back?