Microsoft Security Essentials is a free anti-virus solution for Windows Vista and 7.
This freeware works most of the time invisible in the background. has an On-Access-Scanner, that will scan all files that are processed by the system. Also there is an On-Demand-Scanner, that will check user specified files and folders on your hard-disk. Most of the time this software will work min the background. Just if it find obvious infected files you will see a popup., that will give hints, what you could do.