PC hangs after changing new hard drive and upgrading to Windows 7

I recently upgraded my Vista to Windows 7 and installed a new western digital hard drive. The guy at the shop transferred all my files into the new hard drive and also installed the new Windows. Everything was working fine there but when I reached home, my computer started hanging up after sometime. I checked the hard disk for any weird sounds but there were none. Can someone tell me; is there a problem with the Windows 7? Or is it a problem with my new hard disk? So I can change the damn thing and not risk losing my files.
