do you know where that key is stored ?
what you need to clarify is did you use bit locker to encrypt the whole external drive ?
If you didnt then we need to discover what method you used but if it's windows built in EFS then it wouldnt have worked on an external drive.
If you mean the file is on another disk but that's internal then you might just have encrypted it with EFS which is done by right clicking a file or folder then picking properties then advanced and ticking the box that says "encrypt contents to secure data"
If you can see the file or folder make sure your administrator on your computer and using this tweak "
take ownership" right click on the file and take ownership then you should be able to access it ?
let me know how it goes
if it was bit locker then to unlock the drive you will need the encryption key it asked you to store, without it reinstalling vista isnt going to help.