You can. But you shouldn't. A clean install is HIGHLY recommended. And I recommend Vista 64-bit Ultimate. You can find this on the WinBeta rls if you search around(has all 32 and 64 bit ver). And go to keznews or search around here about recommended ways to activate and so on.
Core i7 920 @ 3.8Ghz | 6GB OCZ DDR3 8-8-8-24 @ 1600mhz | eVga x58 Mobo | 2 x eVga GTX 460 SLI | Intel X25-M + 3x Seagate + WD Black = 2.75TB | X-Fi Titanium | PCP&C Silencer 750 | G15 KB | G5 Mouse | G35 Headset | Z-5500 Digital | Samsung T260HD