A question that is asked many times here on the forum
to be honest its a matter of choice and opsystem will make a difference.
1 antivirus (two will clash)
as much antispyware as you like depending on how paranoid you are:0)
a cleaner and a blocker can also be very useful
Paid for or free?
XP Needs a third party firewall such as ZoneAlarm (I run Sygate) Avast antivirus, Malwarebytes antispyware (superantspyware as a backup or in preference), SpywareBlaster as a blocker and CCleaner as junk file eliminator and lots of other useful tool in it as well including the only safe reg cleaner I would recommend)
Vista / W7 Own firewall are good enough, add the other stuff from the XP list