External Hard Drive can't boot!

I have a Seagate 500GB external hard drive that was formatted on my Macbook at FAT32. I was using it without problem both on my Macbook and my brother's Windows 98 desktop. I tried to use in with my new laptop, running Windows 7. I found out I had to assign a volume letter to it in order to see its contents. I did so, only to get a message asking me to format the drive! Moreover, now I cannot use it neither on my Macbook nor at my brother's desktop... I tried removing the letter but nothing changed; it is still unrecognizable, telling me that no known operating system is installed in it (or something like that)... I guess it's a disk header thing (after assigning a volume letter). Maybe I could fix this with some kind of software?
Any help would be greatly appreciated!
Dionysis.

Nope, both were happy with FAT32.
I would format the drive but it's far from being empty

I appreciate the feedback.
Well, I will copy the data elsewhere, if first I can get the hard disk to be read!
I am working on a data recovery program now.
Well, the reason I was using FAT32 was to be able to use the hard disk both with PC and Mac.

Not at the same time. But I stored data from both, and wanted to read them from both. I didn't use software with the disk.