Firstly prevention is the best cure and following these guides will lessen the chance for viruses and spyware to gain access to your pc, there are some good anti-virus scanners out there including:

AVG
Avast

With spyware i would recommend one or more of these, each one has advantages that the other doesn't have:

Windows Defender
SpyBot
Adaware

The next logical step is to have a firewall which protect rogue computers connecting to your computer via the internet. I could recommend more than one but i think Zonealarm is by far the best.

Basically when a program tries to get in or out of the computer through the internet then you will get a popup asking for permission, once you allow it permission once then you don't need to do it every time. It can be confusing knowing whether or not you should be granting something permission so please read all the documents that comes with it carefully but i try and advise common sense, if your installing a program then you'll expect it to be getting permission requests but if your just reading a web site and not installing or loading something or running an application and it asks for permission then i would be very skeptical.

If your unsure you can choose to block it this time and so if it keeps coming up sporadically then you can choose to always deny permission to this. The problems arise when you deny permission to a program by mistake and then it wont load but as i said use common sense and you should be fine as the benefits of having a firewall far outweigh the negatives.

Windows Update:
Windows Update is a web-based update service that Microsoft provides for its operating systems. You can use it to download system component updates, service packs (SPs), security fixes, free upgrades, and hardware driver updates. You can use Windows Update with Windows 98, Me, NT, 2000, and XP.

When you visit the Windows Update site, your computer is scanned to determine what updates you need. You can then choose which updates to install and how to install them.

A lot of people may remember a couple of years ago all the fuss surrounding the Blaster Worm Virus that restarted your computer every minute or so, funnily enough anyone who had downloaded the latest update from microsoft would have been immune from the viruses microsoft released a patch 1 month previous however the scale on which it spread shows how many people weren't using this free service and also highlights how important it is to keep your system updated.

You must be connected to the Internet.
You must use Internet Explorer as your web browser.
The updates can be large, and may take several hours to download over a dial-up connection.
In Windows NT, 2000, and XP, you must be logged in with administrative rights.