Problem with Hard drive swap?

Hello, I recently got a tower for free (dell optiplex 750) without a hard drive, i have several spares from other units i have had, so i swapped in a WD 500 GB Raptor drive with windows 7 Ult. 64bit already installed on it. powered it up and it gets to the windows splash screen when the colors start spinning, then it freezes and reboots, goes thru the same thing over and over till you do a "hard" shutdown, then it goes into "windows did not shut down correctly, what would you like to do" screen. At that point i went into the system setup when i started it next, went into BIOS and the HD was there and set for first boot priority, so i shut down, grabbed another HD and swapped it in, started it up and this wont even get to the splash screen, so i checked the bios again and this HD wasnt recognized in the boot list. Any idea why??? I know the hard drives are good because i put them in another desktop and it fired right up. I have done swaps like this many times in the past but never had this problem before, any leads on what,where,why would be greatly appreciated.