I vote against AVG 8.0 - lot of problems. At least, until they fix it up.
Free antiviruses - Avast, Avira.
Free firewall - Comodo
Spybot Search and Destroy
AdAware
AVG Anti-Spyware
WinPatrol
Stinger ( on demand Trojan scanner )
These can be installed alongside, since they won't interfere with each other, or AV\firewall.