It depends entirely on the current format of the hard drive. If it's formatted as FAT32, you can read and write from either a PC or a Mac (but there's a single file size limit of 4 GB). If it's formatted as NTFS, on the Mac you can read from the drive but not write to it.
The best format for use with the Mac is Mac OS Extended (Journaled) - that format cannot be read from or written to from a Windows PC.
You can connect the drive to your Mac, and launch Applications > Utilities > Disk Utility to determine the format of the external drive. That's also the application you'd use to reformat the drive as Mac OS Extended (Journaled) if you choose to do so (that process will erase the drive, but you could move your data to your internal drive, then reformat, then copy it back).