Parallels 2.5 to 4

I am having a devil of a time.
I upgraded my MacBook from Leopard to Snow Leopard.
Parallels 2.5 got shoved into a folder called Incompatible Software.
So I bought a copy of 4. 0.
I went to upgrade it and it couldn't find my previous parallels drive to upgrade.
I searched for a .hdd file but there wasn't one.
Luckily, I had a back up so I copied that to my hard drive.
But Parallels 4.0 won't recognize it.
I really don't want to have to go through all the install crap again just to run one program!
Any thoughts, please?
Paul