vista partition won't boot after hard drive transferred to a new PC

Hi recently i bought a new computer at a auction for $150 they said it didn't have a operating system but turns out that its 750G hard drive was broken so i just transferred my hard drive from my old computer over and it worked the old operating system was windows xp pro, media center edition. and while it was on there i upgraded it to windows vista home premium 32x but i noticed that even though this computer had a better cpu dual core. that it still only had 1G of RAM i was just going to transfer the extra RAM cards over from my old computer but then i noticed that it only had 2 slots where as my other computer had 4 and there were all the same size RAM cards so i couldn't do anything and since i like to play games the computer just can't handle it the games i play for its lack of RAM so then i decided to go back to my old computer when i moved the hard drive back i couldn't boot from the vista partition i don't know why. so i thought i'd just install a new one and and transfer the files over but i can't since there isn't enough free space and and the installation CD will only let me browse the hard drive and not delete anything and i have no devices to transfer the files to. but i noticed there was a recover partition installed with a repair feature but when i tried it, it asked me to type in the administrator password so i tried mine of course and it wasn't it so then i tried nothing since the default administrator account doesn't have one which also didn't work so now i'm stuck. and i can't just reformat because i have important file on there i don't want to lose, i have no back up of the files, and i have no device to export the files to with the Installation CD. So now i really need help any suggestions would be appreciated, Thank you.