Windows Security

In order to prevent your Windows machine from fetching viruses, I suggest doing the following steps from time to time:

Windows Update (if you don't have an internet-connection, use Winboard UpdateBuilder or similar)
Check that your firewall is up and running
Check the stuff that is loaded at the startup of Windows and the non-M$ services
Start -> Run (DE: Ausführen) -> msconfig
Update your Anti-Virus tool (nice free ones I know are AVG and AntiVir)
Do the online security check Shields Up -> Shields Up -> All Service Ports
Check if you are using the latest versions of browser, Flash, Java, PDF viewer, etc. Here is a nice online test