In terms of resources required, the lightest version of Windows is 1.0, but I doubt this would suit your needs. The lightest supported version is XP, but that's getting a little long in the tooth now and will soon be out of support.
Personally, I would either
1. Install more memory and use Windows 7. Memory is not expensive these days.
A standard Xp would compete with an antivirus, a firewall and a browser if they had to share 500MB. It Ram size can't improve, W2k would run better than the standard Xp. Observed by myself with both OS on a 500MB machine.
Running an application on W2k isn't tragic... Many do, but you have to check it before.