Is your computer Mac or PC? (I think it's PC right?) If you have a MAC, running Windows OS is pretty easy through Bootcamp. Just backup all of the driver by Bootcamp, create a Windows partition (NTFS or FAT32) and install Windows likes on a PC. If you have a PC, it's quite complicated if you want to run MAC OS. First, you of course have to create a new MAC partition. Then boot by a crack MAC OS DVD (Kalyway for example) and install (quite easy steps). However, the hardest part is behind them. You have to find the suitable drivers for your hardware and it's not easy at all because you can not use the drivers on Windows OS. For the account A & B: You can set up A on Xp, B on MAC or whatever. HOWEVER, these two accounts can not communicate to each other because they are operated by two different system. Luckily, you can still transfer the files between them.