OK - "thread creep" I know but ... Very often a computers' vulnerability stems from an out of date or unpatched software application for which an exploit has been developed. A (surprisingly little known) tool which helps to address this issue is here:
Download - Personal (PSI) - Vulnerability Scanning - Secunia.com
Highly recommended - I have been using it on a homenetwork (XP SP3 and Vista) for over a year now with no problems/issues. They have just produced RC4. FileHippo produce something similar, but it does not (yet) seem to be as comprehensive.