Computer now refuses to boot.

It's Windows Vista, but I don't have a vista disc with me right now.
I was thinking about using an external hdd to boot from but I have no clue how the heck to do that.

It doesn't matter what version of the disc you get (Home Basic, Home Premium, Professional, or Ultimate) as Vista introduced a nifty little feature where regardless of what version you install, you get Vista Ultimate, the key you put in simply determines the features to "unlock".