AVG free as antivirus(realtime),Comodo Firewall 2.4 pro as firewall(realtime,version 3 is better suited for XP and Vista),Spyware Terminator with resident shield to prevent the installing of spyware(realtime),System Safety Monitor Free to prevent the executing of malicious scripts(realtime) and finally Rootkit Revealer to detect if a hacker compromised your system(on demand)
After this setup,your system is pretty good secured.....