Boot Windows 7. Didn't use bootcamp assistant

Question:
How do I boot into windows 7 if i didn't use the boot camp assistant to initially install windows?

Laptop Specs:
MacBook Pro 2011 (purchased stock)
OS X Lion
Intel i7 dual core
4 gigs ram
Internal 120 gig Corsair SSD (Installed after receiving laptop)
External 500 gig Toshiba Apple HDD (Original internal HDD installed into usb enclosure)

I also have a windows pc

Background on what I have done:
I have successfully installed Windows 7 onto a usb drive. It is fully bootable and working while in the usb enclosure. If anyone is wondering how i did this i will give instructions at the following post http://forums.macrumors.com/showthread.php?p=13169519 . I have a minimal amount of free space available on my internal ssd on my mac so i do not want windows on the internal drive. I am trying to boot into windows on the external drive on my mac. Shouldn't it be possible? If it is how do I do it?