I have seen a few people lately that have had their computers infected with virus's , spyware , malware.
There is a common theme in all these cases apart from the users behavior that caused them to become infected.
They had not kept up to date with Windows Update.
Windows Update is your first line of defence and automatic updates should be turned on.
Why are Windows Updates so critical to the prevention of virus's and infections in computers?
Virus's are written to exploit vulnerabilities in the operating system.
Windows Updates are released to amongst other things patch vulnerabilities in the operating system.
Therefore it makes sense to keep Windows Update's automatic update turned on along with your anti virus solution so that they can assist keeping malware etc at bay.
However remember it is also your actions that can help you not get infected as I have previously posted
A myth that floats around is that if you have your operating system on one hard drive and your documents on another hard drive you will be immune to your documents being infected by virus / malware infections.
This is wrong.
If your computer can see the hard drive then so can the virus.
Virus's can spread over multiple hard drives and networks ( worms ) .
Remember virus protection starts with your behavior.
No amount of hard drives can protect you.