WD HDD not opening

I have checked the internet high and low and i got NO where to find answers..SO..

Short and simple:
i have a 1TB WD HDD.
Im working on my desktop Vista machine.
I use my WD HDD here and their to back up my videos and work.
I had to reformat my desktop back to Vista from Win 7 because of a bad product key and it was getting slow.

After reinstallation of my computer i plugged in my WD HDD and nothing is working.
The computer sees the HDD but it wont let me open it. its saying to reformat it. Obvisly im not going too because i have 800Gigs of data on it..

Funny thing is, is that the WD works perfectly on my laptop (win 7) but not my new installed Vista desktop...

Can someone Please help me on fixing this problem??
And YES, i have tried opening the properties, selecting a different letter for it, doing checks and all the above as far as i know...