AVG Anti-Virus Free Edition is a free anti-virus protection tool developed by GRISOFT for home use. It does not use up lots of system resources like Symantec or McAfee, so your computer doesn't slow down. It has an automatic update that can be scheduled. I have personally used it find viruses that the so called big names (see above) cannot see.

If you want a package that offers protection from spyware, try AVG Anti-Spyware Free.