Just now i successfully install BackTrack 5 R3 64-bit GNOME on my computer dual booting with Windows 7 64-bit. For installing this new BackTrack,first you can get OS file from here(BackTrack download page)
Select option which meet with your system. For example, im select BackTrack 5 R3,GNOME,64,ISO,Direct..Then click on "CLICK TO DOWNLOAD"
After download is complete,burn ISO file to DVD. you can use Nero to perform this. REMEMBER to burn as image file NOT DATA FILE
Setting your BIOS(if needed) to set CD/DVD ROM as first boot then boot BackTrack from DVD. Just press "Enter" and "Enter" to start BackTrack as "demo session"(can install later.please continue to read)
After done boot BackTrack,type "startx" to enter GUI session on BackTrack 5 R3. You should see "Install" file clearly on the desktop. Double click it to start installing BackTrack 5 R3 64-bit
Choose your preferred language(mine select English)
Choose time zone(select your time zone.can change later)
Choose keyboard layout(or just next for default keyboard layout)
Now,This is most important part.please be careful from here. Choose your partition to install BT. What its need to work is EXT3 or EXT4 file system(mine is EXT4,8GB above partition size,partition for swap(if necessary) Select "advance" for custom partition choice.
Make change to meet with what you needed. From picture below,i already have 20GB EXT4(my previous BT)