Hi,
Its not so much that Win2K Pro wont mirror the system partition, as Win2K Pro wont mirror at all!
The first software option you have is to move to Win2K server (or Advanced server)
I found a company "Winraid" that delivers raid technology to Win2K
http://www.winraid.com/
And here's another one but they want to sell you an enclosure for your disks as well - although its software implemented raid...
http://www.adstech.com/media/mediafiles/PR-PYRODVRAID.htm
(remove spaces from the above URL)
...but if it was me, I'd be investing in a raid controller card. Why waste CPU cycles and slow your system down when you can let a card do all the work?
At least compare the prices of the software to the prices of raid controller cards before making your decision...