I'm still on XP SP2 (I refuse to Update to SP3) with .NET 3.5, and Mids doesn't falter at all.
I alternate between Maximized and not pending on whatever I'm doing at that time, but I've yet to see anything short of a Lock-up from the Program at all. (Yes, it does Lock-up sometimes, rarely, but that problem is on my end, not the Program itself)