I bought the Game via Steam, installed all episodes and got through EP1 just fine, then I tried to play EP2 and the trouble started.
EP2: The game keeps crashing shortly after the Police takes Doc Brown into custody. Precisely as the camera focuses on Marty peeking out from behind the DeLorean the game crashes. Deinstalled the game and redownloaded it, no change.
EP3: Shortly after starting a new game, after the "Telltale" credit rolls, it crashes.
EP4: same as EP3
EP5: Was able to start it, but shortly after grabbing that electric device it crashes as well.
ADM Athlon X2 5200+
4 GB RAM
Radeon X1950Pro (512 MB VRAM, Recommended Drivers 10.2)
Windows Vista SP2, most(all(?)) recommended upgrades installed.