You might think that with all of today’s security measures for your computer, that massive virus outbreaks are a thing of the past, but you would be wrong.
Right now, there is a worm called Conficker (also called Downadup) that is exploiting missing Windows patches. What Conficker does is allow other infected computers to attack yours, which would allow your computer to be taken over in turn and used to attack yet more computers.
Conficker has also been seen to allow downloads of fake antivirus programs that attempt to scare you into buying the fake program, when then gives your credit card number to criminals.
Windows Updates are an important part of keeping your computer healthy. I know that sometimes the update notifier can be annoying and a nag, but it really is important that you let your computer be updated.
If you are in the middle of an important project, let it go until you are done; but then find a time when your computer is not busy and install the updates.