Before deciding on any computer consider what you will be using it for, what software you will be running, and how long you expect to use it along with your future needs. Consider the system requirements of any software package you intend to use. When considering an operating system, think of security as well as the length of support you will receive from Microsoft. By support, I mean operating system updates. For example, they no longer release patches for Windows NT or 2000.
Personally, you couldn't give me an XP computer with less than 1 GB of RAM or a Vista box with less than 2 GB of RAM, and even that can be painfully slow depending on what you're doing.
I agree with the above comment recommending Kapersky antivirus. I've been using it for years. Symantec and McAfee are resource hogs, and I've seen McAfee completely trash workstations, and I had to resolve an issue with McAfee's Enterprise Policy Orchastrator last summer after it brought down our domain controllers bringing our entire network to a halt. Over 500 people got to go home after half a day.
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edited to say, oh crap! I just noticed this thread was a year old.