I don't know if there's one 'best' program to use for each, since everybody has a personal preference. That being said, here's my list:
Antivirus:
avast! Antivirus Home Edition - about 13 MB of memory
Firewall:
ZoneAlarm Free - about 18 MB of memory
AntiSpyware: None. If I had to choose, either
Spy Sweeper (about 10 MB of memory) or
Microsoft AntiSpyware (about 20 MB of memory).
I think you're fine in terms of antivirus and firewall. If you want to go for antispyware, Spy Sweeper uses less memory, but it costs about $30. Microsoft AntiSpyware is free, but uses more memory.
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