Is that firmware in your pocket, or are you just happy to see me?
Well, a Hard Drive, is a Hard Drive, is a Hard Drive. Pins, speed, cache, bus type etc... but either will work on a Mac or PC. But the only issues with the case might be, is the case designed for the hard drive type itself. (pins, rpm speed, physical size, etc).
If you mean the formatting of the hard drive, then the Apple formatting system may or may not be recognized by your computer depending on what Apple format was used on the drive. Some have a mix of OS 9 and OS X.
If it is an earlier OS type like OS 8 or OS 9, then more than likely it would not mount on the PC, and the drive would have to be formatted to a Windows compatible partition and file structure. (all data would be lost)
If it's a later version of the Mac OS like OS X.1 (10.1), X.2 (10.2), or X.3 (10.3) then it may work just fine with out reformatting.
But I'm 99% sure that no matter what formatting is on the drive right now, if you install it in the case (assuming the case was made for laptop style drives) you could reformat it with a Windows Hard Drive program and be all set. (all data would be lost)
One format you may want to use is an old FAT32 that can be read by both Mac OS X and Windows, if you need to use it on both systems. But keep in mind, the 2 GB file size limit with that format.
Good Luck.
-Blackstar