How to use encrypted laptop hard drive ( hd ) in external usb case?

Hi,

I have a dell d830 with disk encryption (the hd is seagate momentus something) and I replace that HD to a new one. 

I mounted that HD in an external usb sata casing and now I want to read its contents, what do I need to know to "desencrypt" it? I know the password, but it seems I need some software to make it work, what is it?

I have win7 64bit.