Windows 7: 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.

Sounds like you're going to have to reformat the drive (on the Windows 7 pc). I had thought there were differences between the Apple and PC filing systems.

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

If you can - move all the stuff to another hard drive, reformat it, and then move the stuff back. Any reason why you are still using FAT32. NTFS is more recent and reliable.

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.

Thats a good reason for using FAT32 Dionysis. So you're using the same external disk for both Apple and PC programs at the same time ???? I'm a little surprised you didn't have problems doing that as the Apple and PC programs are not the same (nor are the disk os'es). Maybe thats why you're having boot problems now ????? Who knows, but good luck with it

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.