Find a spare IDE cable that's on the ribbon you are using for your HD at the moment, attach the old HD along with a spare power cable from the PSU, and ensure that the little jumper at the back of the old HD is in Slave position and the running HD is in Master. Then boot up, it should work.
A note of warning - if you're data is in a profile's documents that had a password on it, I know of no way to get that data out (dunno if anyone else knows how to). This happened to me and I lost a ton of data because I couldn't turn of the privacy function of the profile