Is there a virus scanner that can be downloaded from the Internet free of charge to protect computers from being infected by a serious virus? Thank you!

There 3 that come to mind....
1 - AVG Antivirus
2 - ANTIVIR Antivirus
3 - AVAST Antivirus

Good Luck

Of the 3, the Avast! and AntiVir programs I'd go for. The AVG program is good bar the updates are erratic in recent months
Think also about what spyware detection/removal program you require?
If anything these are more prevalant than virus's
Adaware SE and Spywareblaster are both free and do a great job But keep them updated once per week
And consider a firewall (There are several free) this will give you an extra line of defence.
If you are contemplating using Broadband, this goes to the top of the security program shopping list Won't last 10 minutes without one