Beware if anyone sends you a "Virtual Card" or a "Postcard from Hallmark" via email, especially if you weren't expecting it. It will most definitely be a surprise. This also a major problem.

The biggest threat to computers are not Viruses as is widely believed, it is Spyware and AdWare. All companies need to advertise but some do it without your knowledge using your system resources to "Spread the word".

Of late I have attended to many computers with quite serious Virus infections. Please ensure that your chosen program always runs in the background and run a full scan every week. Keep backups of all important work. Don't backup before a scan. It's a bit pointless backing up the virus. If one backs up to a portable Hard Drive, scan that regularly as well.

There are 5 Free Anti Virus programs that each have their own champions. Only 1 Anti Virus at a time though.
Microsoft Security Essentials Tested O.K.
Avast V5 Free In my opinion, The Best.
Avira Free Very Good, but make sure it updates.
AVG Free In my opinion, Not one of the best.
Firewall + Anti Virus by Comodo Untested.

Trend Micro offer an on-line scanning service which is highly respected.

CCleaner, Will show a list of everything it will delete after confirmation. NOTE: If you leave the Security Setting selected all traces of your Internet usage will be removed, including all history traces.

There are others like ComboFix, HijackThis and MalwareBytes but if you are inexperienced then leave them alone.