please help load vista harddrive on xp computer

this is what i am trying to do i have a hard drive from my old acer aspire computer t690 it has vista loaded i installed this harddrive into my hp dx 5150 that has xp pro on it.the computer regonizes the hard drive if i boot using xp but it wont let me boot using the harddrive that has vista installed.i have tried running windows auto repair and was not able to get anyware can some one guide me in the right direction please. oh ya if it helps they are both sata hard drives thanks in advance for your help..justin sitterly