PC
Windows Vista has alot of bugs, dont buy it yet. My sister bought a laptop for like 500 euro and after a day she cant even start windows vista without letting it crash!

My father has Windows Vista too (the ultimate or something) it works but you cant use old printers and stuff anymore.
Anyway dont buy it. It needs alot of fixes, Windows XP is better right now!
MAC
Well I never used it, but its faster and does things more selfish like:
PC:error you need driver, download one
*download one*
PC: Error wrong one please download another one
*you download another one*
PC:succes! ERROR you cannot run this program, you dont have dxd9.dll
*you search on google*
*after a hour you know what it is and you download Directx 9*
PC: ERROR the computer doesnt support this. Video card too low, CPU too and RAM.
MAC:You dont have the system requirements for it, continue?
MAC:Error you need driver, please click here to download it!
*is downloading...*
MAC:Ok it works *loads* You dont have Directx 9. Please click the download button below to download it.
*downloads*
Anyway PC!
