While I agree with the above post to some extent. In theory, it should be possible to do a native installation of Windows XP on one of the Intel Macs, http://store.apple.com/1-800-MY-APPLE/WebObjects/AppleStore.woa/71002/wo/1o37Jif38fct2gKaF98ZMivKbT4/0.SLID?nclm=iMac&mco=DD2F223D which feature a dual core 1.83GHz Intel processor. While Apple does not officially support Windows XP on their Intel based computers, it should be possible to install it.
If you have a PowerPC based Mac (if you don't have a Mac I would get one of the Intel ones) but if you do, you can use an emulator called Virtual PC that runs within Mac OSX (the Mac operating system) and allows you to install Windows XP and use it just like a Windows desktop at a slightly slower speed due to the emulation and translation of the x86 instructions that has to be performed.
http://www.microsoft.com/mac/products/virtualpc/virtualpc.aspx?pid=virtualpc
I do agree with the Linux distro, Yellow Dog, it's the only one I know of that is available for the PowerPC platform but, as I said above, if you have an Intel based Mac, it should be possible to install a x86 distro of Linux on it just as easily. But the Intel Mac would not work with PowerPC distros.
[update:] It has come to light that you CANNOT install Windows XP on the new Intel Macs, read the following for more information, http://www.betanews.com/article/XP_Wont_Run_on_Intel_MacBook_iMac/1137003330 and the same probably goes for x86 Linux as well.