IMHO Windows 2000 Pro
The Windows 9x series don't handle memory issues very well, aren't
stable, and don't handle large hard drives well.
Windows XP is basically Windows 2000 with lots of eye candy. It's OK, but
I find it's more for home use then anything else.
Where as Windows 2000 Pro can be as "home use" or "commercial use"
as you want it to be.
It's been out long enough that drivers are out there for whatever you
need to get running in it (assuming the company still exists).
It handles large capacity hard drives very well.
It has built in RAID features allowing you to use multiple hard drives
linked together as one large one.
It's networking is very solid and relitivly reliable. So if you have to connect
it to other PCs in the house (or studio) it's really easy and solid.
Overall Windows 2000 is fast, reliable, and handles memory and crashes