MS has produced a free anti-virus program. It rivals the best- which are Avira's Antivir and Alwil's avast! There's a nice review here, based in part on detection rates, where MS, Antivir and avast! do well. Antivir comes out on top, but largely due to avast!'s counter-intuitive scanner GUI- which is due to be replaced when avast! 5 comes out very soon.
One point to note is that if you have set up limited user accounts for family members, those limited users will be able to disable protection if you use Antivir, because the limited user password setting is only available in the paid version; with avast!, you can set a password so limited users will not be able to disable AV protection.
UPDATE: More on the recent test of detection rates here.