You reinstall Vista on this machine, using a valid Vista DVD (that's a retail version with an unused license code, not an OEM version from your old computer). Be sure to choose the option to install it on that second hard disk and make the system dual boot.
Windows 7 would probably work also, and that might be a better choice.
okay so here goes. i have an xp system. i use it for gaming. unfortunately, battlefield 3 will not run with dx9 which is on xp. But, i have a vista hard drive, taken from another computer, so my question is this: can i run a dual boot? there is one problem though. one day, i started up my computer (now keep in mind that i have both hard drives connected.) and it booted to vista. but before it started up on vista it always displayed registry files or something being fixed/repaired. im no computer nerd but i do know my way around. no matter what i did in bios it wouldnt boot xp. it also didnt show the xp hard drive. so i was forced to unplug the vista hard drive and everything is now fine. i just wanted to know if there was a way for it to stop doing this and if i could somehow setup a dual boot system for when i need to game with direct x 11. i was also wondering if there was a way for me to reconfigure vista to the computer that it is in now rather then the old computer that i pulled it out of.