Check MD5 Hashes
ISO:
BT5-GNOME-32bit - b01a93a916fabb6d1640bd0054428e17
BT5-GNOME-64bit - 75c4e7a969abc873d9e085656b156345
BT5-KDE-32bit - 4150643026d292717f77ebb83948a034
BT5-KDE-64bit - 80d65610de90ac7ede49b9f7935dfdd2

Download:
http://mirrors.rit.edu/backtrack/
http://www.backtrack-linux.org/downloads/

Tuts and more info:
http://www.backtrack-linux.org/forums/backtrack-5-forums/

A quick and simple guide to install your BT5 on a live USB drive.

Plug in your USB drive (larger than 2GB).
Format it to FAT32.
Download Unetbootin from http://unetbootin.sourceforge.net
Start Unetbootin and select "Diskimage" ( BT5 .iso file)
Select your USB drive and select "OK".
Exit or Reboot.

Easy Install BackTrack "Persistent" to USB

1 choose usb key
2 choose iso
3 choose much space you want for persistence
4 klik create
5 wait till done
6 enjoy your persistent usb key
Download
http://www.linuxliveusb.com/