Grisoft still offer the free versions of their anti-virus and anti-spyware software on their website. I have this working fine on Vista 64 - no problems whatsoever with install or operation. This doesn't give you a full function firewall though, so make sure you manually configure the Windows firewall in Vista to disable all but the essential inbound traffic, and turn off any unnecessary protocols on your network card too. (and/or get a third party firewall product)
I tried McAfee and ZoneAlarm Internet Security Suite but neither currently support Vista 64.