Installing old HDD with operating system.

I recently finished building a new computer system to replace my old HP pre-built pc. i was going to sell the old HP but i decided to strip it and try to reuse as many of the components as i could. My new system has windows 7, and im trying to install the old hard drive that has windows XP on it. can i do this without having to reformat the HDD and without reinstalling windows XP (which i dont have a copy of). thanks, any help would be greatly appreciated.

what exactly are you trying to do with the drive? use it as additional storage? And do you not want to lose the data already present on the old XP HD? If you just want to access the drive and make it into usable storage, you just have to plug it in to your new computer. It gets a bit trickier if this is an older EIDE drive and not SATA, but either way, not too difficult. Once installed, just take all that data you want to keep, and store it temporarily on your new drive. Reformat the old drive, this will get rid of the old OS on the disk that is not necessary anymore.

Though before you do that, I'd recommend recovering your windows XP key. You may not need it at all anymore, but you never know when you'll want it. With Windows 7 Professional and Ultimate, you'll need a valid XP key to use XP mode for example. The easiest tool to extract the XP key is a utility called siw. It just has to be run on the computer while it's on. There is a way to get it off the hard drive directly when placed inside a different computer, but it's much more involved.

Thanks for the quck reply. What im trying to do is to be able to use Windows XP thats installed in on the old HDD with my new system. there are some websites i need to access for school and it doesnt allow me to access them using windows 7. so i figured if i could install the old hard drive i could boot off of it and be able to use windows xp instead of 7, and after im done with XP, i could just boot off of the new drive again. Hope this clears it up. Im not looking for addtional storage.

Yes your cpu must support vitalization to use xp mode, most semi-moderns cpu's do.

Most likely it is your browser that is keeping you from logging into school, while I use FF i have found it is always best to log into school using an IE version at lest one if not two generations old.

What you are trying to do is called a dual boot. Google will be more than happy to tell you how to do it.

To do a dual boot, you will need to extract the key as mentioned before. You will also have to completely reinstall windows xp on that old hard drive since your hardware is vastly different from your old HP system, to your new build. It might manage to boot, but it probably won't be stable, or run anything you need it to run unless you reinstall XP on that drive. As to the specifics for doing a dual boot, it's not too difficult, though I'd recommend looking for a Vista / XP dual boot guide on the internet. There probably won't be any Win 7 / XP dual boot guides, but they really should be the same process.

If you are going to all this trouble for the sake of a few school related websites, they really should get cracking on making them work. Vista has been around for years now, and any type of web content that works on Vista, should also work on Win 7. If your school hasn't gotten around to supporting their stuff on Vista, they really need to get going. XP is not going to be supported forever.

I would focus on trying to get your build with Windows 7 to access the sites you need instead of messing with trying to get your computer to boot off your old HD. If the school site doesn't come up with Firefox, try it in IE. IE8 has a compatibility mode that you can use for older sites. I HIGHLY doubt the problem is with Windows 7. Like was mentioned above, it's most likely just a browser compatibility issue.