The biggest threat computer users face on the internet is a virus.
Even though there are other threats such as spyware, adware, trojans etc, the reason why viruses are the biggest threat is because they are created to crash/freeze your computer, destroy your documents, and infect other computers on your network.
One of the best way to scan for a virus is to first boot your computer in safe mode before performing the scan.
If you boot your computer in safe mode it will not run any other services apart from the Microsoft Windows services.
This will make it very easy for the antivirus scanner to pick up all the files that belongs to the virus on your computer.
The second best way to perform a virus scan is to first restore your computer to a date in the past.
Windows XP and Vista has a built-in computer restore utility that can restore your computer to a date in the past, such as yesterday, a week ago or even past 3 months.
You should restore your computer to a day in the past you were sure you did not have any virus problems.
Once your computer has been restored you should then perform a general scan to detect if there's any virus on your PC.
Another way to best scan your PC for malware is to scan every folders on your computer.
Don't exclude any files on your computer, allow your antivirus program to scan through every folders on your computer.
This is because virus/spyware developers sometimes find a way to hide virus in file locations that you might think are safe.