Games not running after moving files from old pc

I have a friend who got a newer pc and moved the files from his old computer on to it. He told me he just plugged in the harddrive from the newer one into an open slot on the motherboard of the old computer and moved over the files. Everything was going fine until he tried to play some games, every time he tries to run one he gets "game.exe has stopped working" and this is all games apparently, from the ones that came with the pc to one he got on a new install of steam. However it only seems to effect games as the pictures and music from the old one open fine.

If it helps the old pc was xp and this one is vista.

Any input would be appreciated.

That's what he did, he had no games on the other pc. All the games were on the new harddrive that came with the pc; solitare, minesweeper etc. And one he installed and tried playing off of steam yeilds the same error.

Is the new PC actually new? If so, did he build it?superclocked

Nah, he bought it from someone else, they cleaned it out of pretty much everything so it was basically like out of the factory, thing is though all the games worked before the files were moved from the older pc to this one.

Pretty sure that was all done before he bought it, but I guess I'll tell him to check that.