Its WIN2k "Professional", pretty much designed for the business world. IMHO WIN2k is very stable and fast, by changing the styles or installing third party software is just going to slow down the system or even make the system unstable. I love the way WIN2k the way it is.
		
		
	 
I used 3dmark, since it's easy, to test with when I was messing w/ my shell and all.
WindowsBlinds: yes, and I don't use it.
Litestep: no (+~200 3Dmarks, FYI; 9675 being normal in explorer)
MBM: yes (-~100 3Dmarks)
Rainmeter: no change
It's all the stuff that integrates into Explorer that'll screw it up (I've used a lot, just to, in the end, get a command prompt open in the current open directory in explorer). I can't uninstall half my programs correctly now (no uninstall apps and add/remove programs doesn't work), I can't get to my NIC's properties outside of the command line, the folder sidebar in explorer doesn't work...after that it's little stuff.
...but it's still fast and stable as a rock, which has kept me from reinstalling. One of these days I will...