My friend is the IT guy at my old company
AND owns a  LAN gaming center.  He recommended
these programs to me.  I have had nothing but
good experiences with this software.
Avast is a great program that constantly updates
itself.  It works in conjunction with MS Office and
Internet Explorer.  It is worth registering, because
then the program works for much longer.  Make
sure you download the "home" version, that is the
free one.
Click here to download Avast AntiVirus
AVG Recently stopped being free.  Too bad.  Follow
the link to download their trial version.
Click here to download AVG Antivirus
Spybot is a good spyware destroyer that will get rid
of any outside advertising programs that have
become imbedded in your computer system.  It also
protects you on the web and updates constantly.  I
have personally been running it for three years.
Click here to download Spybot Search and Destroy
CC Cleaner cleans up temporary internet files, plus
it cleans up leftover files and registry entries after
software installs and uninstalls.  It is a good way to
keep your machine in tip top shape.
Click here to download CC Cleaner
Ad-Aware looks and functions great.  Like Spybot,
Ad-Aware is a spyware remover and registry
cleaner.  It picks up some things that Spybot leaves
behind.
Click here to download Ad-Aware 2008 Free