Best way to load the OS on a home built?

Hi all, I'm new to the forums and have question. My girlfriend has an old HP/compaq desktop. I am building her a new system with new and used parts. It should be pretty nice when I get it finished. My question is this. The system she has now has windows XP Home loaded on the C: drive and has a backup version on the d: drive. It's one of those deals where it will do a restore to the c: drive if directed. What do you think the odds are that I'll be able to use it (legit version) to load a fresh install on a new hd? Any tips on the best way to do that? The old system will be junked with only the OS reused if possible. I don't want to buy XP or Vista and Win 7 isn't here yet. I'll replace it with Win 7 when the time comes. Thanks in advance.

You wont be able to transfer the operating system from a prebuilt, it is generally tied to your motherboard. You will need to buy vista with the upgrade coupon to 7 for now since the windows 7 RC downloads ended a couple days ago, i suggest 64 bit vista so you can use more ram in the future.

Is there some part of the install programing that looks for some indentifier on the motherboard or is it the hard drive. I don't reall want to reuse the old drive as it is a veeery slow one (old people slow). I do have an OEM copy of XP but it is on the system I use. I know there is some "can do" out there, lol.

I will add that I did get a copy of win 7rc with the idea of installing it on my system when I recently reloaded it. I was not able to install it as it wanted some special cd/dvd driver, that is a well documented bug that has no good resolution posted anywhere. Since I wasn't able to get it going on my system I just set it aside. perhaps I'll give that a try. Does anyone think that the recovery disks will work?