After install Virtual Box
1.press new machine.
2.click on Linux and select Ubuntu 64 bit or Ubuntu 32 bit if u have 32 bit pc .
3.choose amount of ram i suggest using 2gb.
4.then create new harddisk and select how big u want it to be.
5.then press power on and choose the place where u downloaded backtrack 5 and then thats about it and later if u want to, u can install it onto your virtual harddrive.
6.just enter startx when the command line page comes up.