That is very odd to say the least.
I have many hard drives, and Vista actually powers them DOWN incredibly well when they aren't in use (very annoying when I have to wait a whole second for the drves to spin back up thogh
)
I can't tell you what your problem is, but I knwo what can!
In the start menu search box, search for Performance and Reliability monitor. Run it. You will be able to see a resource overview and underneath it specific items including Disk. Open up the disk section and it will state the PID of what is using your hard drive constantly. Use the PID to trace the process to it's original program in task manager.
Once you have the program name, if you don't know what it does you can google it, or reply here and I can help you out.
If any of this needs more clarification let me know (at work so have had to rush this post
)
Oh, and
crabby