how do i boot my xp from this harddrive?

ok i bought a windows xp upgrade and accidentally formated my computer harddrive so the installer would say "cannot find previous version of windows" so i put my hard drive into another computer and then succesfully installed it but now i need to get it back into my other one, the windowxp files are all on it but when i boot it up it sais "invalid disk Replace the disk and press any key" so now the only way i can boot up the windows on this harddrive is to have the hd from the second computer with the first. and also when i try to run the winxp instalation on my first hd it still sais "cannot find previous version" is there something i have to configure to get it so it can run without the second hd?

you bought a upgrade, you need the actual OS

yea first back-up all your files find the original windos os you were using install it and then upgrade

~moved to help~

i found out one mistake i did i only made an update on the the first hd now iv put the second hd back in and booted off the second then used the windows cd and this time i created a new instalation instead of the upgrade so now i have all the files but it sais "missing NTLDR press ctrl,alt,delete to restart" so im guessing im a little closer to getting it to work but can it still be fixed without buying the full version?