good sir/madam

im new to backtrack 5 and want to try this.
laptop specs:
Acer 4755G i5 NVIDIA GeForce GT 540M with NVIDIA Optimus Technology 2g with 4g RAM
OS WINDOW 7 64bit
first i download the backtrack 5 (BT5-KDE-64.iso) at the backtrack 5 website and Install Backtrack 5 Live to a 4gig USB
using unetbootin.when i reboot and booted the usb. before i can reach the stage to type the command "startx" a blackscreen happened. and i format the usb stick '32fat' and install backtrack 5 again and still my pc stops.
can anyone can help me with these.

thanks.