If you’re heavily into Video and/or Music productions go with a MAC (what they excel at)
However for everything else (Including general Video/Music production) go for a PC (Unless you like the sleek looks of a MAC) as there is vastly more and cheaper software/hardware available, that can easily be updated if required. (At a fraction of the cost to upgrade a MAC)
In addition a PC with the same performance as a MAC will probably cost about half the price.
As to OS, then XP Pro is fine, but Windows 7 blows it out of the water, (what Vista should have been) and is due for release in October. (The Beta test version of Windows 7 left the public release of Vista for dead)
Hope this helps

Bill