External HD not being detected by Windows

[Yodel]

I dual boot my iMac, and have found that my Windows partition (70 GB of a 500GB hard drive) is insufficient. A while ago, I picked up a 500GB Seagate external hard drive to store large files from my Mac partition, and would like to use it for the Windows partition as well. The hard drive works like a charm on my Mac, but it doesn't work so well on Windows.

I can only find some sign of its presence in PC mode by going into one of the hardware configuration windows, but it is completely unusable. It's detected, but I can't do anything with it. It doesn't show up as a separate drive in the "My Computer" window.

Any suggestions on how to use it?