SF2 runs on XP... but one of the big gains of SF2 is that it runs in DirectX10 by default, which takes advantage of new shader technology to make the game look better AND run faster.
I run an old Athlon 64 3800 single core (with a modern Radeon HD 6870). The jump to Windows 7 and originally a Radeon HD 4890 with the necessary shader support made a significant improvement in frame rate and looked better too.
On XP, SF2 may run even slower than the original series because it has more of everything and no DX10 shaders to speed things up. But I had SF2 and did play it just fine with XP until I committed to Win 7 and the gpu upgrade.
same here
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System Vitals: Intel Q8200, GTX560, 4GB DDR1333, Win7 64bit
Sims: Race '07/GTR Evo, GTR2, GTL, StrikeFighters2 (complete series), F4:AF, EECH, IL2:1946, Arma2+OA, X-Plane, MSFS X
Games: Skyrim, Mount&Blade:Warband+F&S, Fallout 3, STALKER COP, Medieval II