I Want My XP to work after removing it from my Mac

As long as you have a valid copy of Windows, I don't care what version it is, and you have the license and key code you can install it on any computer you wish. The prerequisite, of course, is that you may only have one instance of the OS on any one computer at a time. If you have ten licensed copies of Windows you can have it on ten different machines. If you have one licensed copy of Windows then you may only install it on one computer.
My experience has been that as long as you have gotten past Microsoft's scrutiny and successfully installed Windows on one machine then successfully installing it on another machine is no problem. I have two licensed copies of XP and have successfully installed one of those copies on two different machines. I have also done the same with MS Office on many occasions and the scrutiny of Office is as tight as it is for Windows.

Although registering by phone is kind of a pain in the butt, it takes a bit longer, what with Microsoft's validation crap I've found that the registration process goes much more smoothly when done by phone. I just don't get the same hassles registering by phone as I do online. I found this out when I had to register both Windows and Office for computers I was setting up for other people, without active internet connections, and in some cases the software was being transferred from old machines to new machines. So while it takes a bit more effort phone registration does go much more smoothly than registering online.

I've never used MAC, but I think the suggestions to upgrade your MAC OS are very good. If you decide to do that then just do a clean install and XP will go bye - bye. If you can't afford to upgrade, or can't upgrade due to system limitations, then just do a clean install of your current OS. Clean install just means you wipe everything off your drive before installing the OS.

But I wouldn't worry about being able to install XP on a different machine. If it is a valid licensed copy of XP then you can install it on whatever machine you wish, but I do suggest registering by phone; much less of a hassle, in the long run. If you've successfully installed it on one machine you shouldn't have any problems installing it on another machine; I never have - to date.

This is my experience, but we're dealing with Microsoft here, and they seem to make up new rules as they go along. I hate dealing with Microsoft and any time I have to update anything from Microsoft I know it's going to take a while. I recently updated Framework and the update process was painfully slow. If you're not old yet you will be when you get done updating anything from Microsoft; I hate it.

  Wild Weasel Keeper

You should upgrade to Mac OS X Tiger, or maybe even Leopard if your Mac computer has Intel chip or PowerPC (PPC) with a processor that's at least 867MHz. Panther is well-known for being slower than all of Apple's latest operating-systems.

Microsoft's Vista operating system runs slower than Windows XP, so if you had to choose between 1 or the other, I would suggest you use Windows XP, rather than Vista (both of which have security-holes).

You're safer with Linux (which is absolutely free, and it can work on PC & Mac computers!) Try OpenSUSE, Fedora, or Ubuntu 8.10:

http://OpenSUSE.org (Free)
http://FedoraProject.org/en/get-fedora (Free)
http://Ubuntu.com (Free)

Before downloading either of those operating systems, you should definitely find out the specs of your computer (like whether it has Intel or PPC processor).

Each of those Linux distributions (distros) allow anyone to download their Live CD's. If you decide to burn the OpenSUSE, Fedora, or Ubuntu ISO's (a.k.a. "disk-images") to CDs, you will be able to test any of those operating systems (after you insert either one of Live-CDs, which you must have previously burned to a CD).

The Microsot OEM license (I presume that is what you have) dictates that you can only install it on one device. If your PC dies, so does your license. It's Microsot's way of making lots of money.

Each time you install or reinstall XP, it will want to authenticate with Microsot. To prevent piracy, they say. However, you can usually install the same copy of XP on different hardware up to 3 times without having to talk to Microsot by phone. This may be necessary if your motherboard blew up, or your hard drive, or whatever. So they give you a few chances. After that you have to talk to them by phone and explain why your hardware packs up so often.

Moving your license from Mac to PC is too big a change (all the hardware will be different). I therefore believe it will be illegal (according to the license agreement, anyway).