The normal file system is NTFS and unless the user did something like lock it down with EFS or shudder, some third party encryption the only limitation I've encountered is the drive may be READ ONLY. Hidden files may be inaccessible but back to some PC and remove that and back to the Mac.

http://www.lostpassword.com has a tool to deal with EFS key loss (which anyone that uses EFS knows to backup that key.)

http://www.drivesavers.com can deal with most other encryption so call them up if you don't know what you used.
Bob