Operating System

U can try connecting the old HD and boot up but usually with XP u can't.

A bit more info here, why did you rebuild? What do you mean by recover your old OS, do you want to run your old version again, copy files from it, what?

Did you build a new system and want to use the old installation of Windows?

More info-I built my computer at an on site classe at my last employer. I tried upgrading my RAM, now my tower will not boot. I've troubleshot everything to RAM, AND CPU. Assuming problem is either RAM slot, or CPU, I was able to replace Mb and CPU. All of this because I already had 2Gigs RAM. Soooo if reuse my hard drive in Master position will my XP po, my last operating System, boot up

I take it you tried using the old RAM again and it did not boot even with that?

If that's the case, my money is on the motherboard and not the CPU.

If you replaced the motherboard with the same model as the old one, or at least the same chipset model, you should be able to boot up just fine, with maybe having to setup some drivers. If you got a totally new model, you may have issues. Try booting into Safe Mode first and see. If you run into a big issue, the PC won't even recognize the drive as bootable.

Thanks hang-the-9. At least I know how to proceed.