Can I take the hard drive I have installed XP on and out it in a new computer and Boot and run it there..
it will work to some extent but it is not advisable. The XP you preinstalled in another computer most likely do not contain the drivers for your new motherboard, vga, lan, sound etc. So you may face some conflict or blue screen errors. Unless you are replication it to another computer with the same motherboard and hardware, you'll be better off installing it on the new computer. If it is a fresh installation of XP without subsequent drivers installation, that should not be so much of a problem. Haven't exactly tried transferring a pre-installed XP HDD over to a new computer so does anyone have anything else to add?
XP will usually blue screen if you try anything like this. It also has copy protection to stop people pirating Windows this way so your installation may well decide it's going to deactivate itself.
It might work if the computers have similar hardware but I wouldn't like to wager on it. It's generally better just to reinstall from a cd then copy any important files back across. It'll leave you with an OS which is much happier as well.