If you are using WinXP you can set it up easily for automatic update, and it is strongly urged that you do so. If, for some reason you don't want to automate it, you should check for Windows and Explorer updates at least every week or so, or about as often as you update your AV signatures.

The vermin can read, so every time a patch comes out, there seems to be a flood of assaults against the hole that's patched, in the hope of catching those who haven't loaded the fix. Updates are easy, and really helpful - and your AV won't have to work so hard.

John