Yes, you can use your ex-PC hard drive with your Mac, the solution was given earlier in this thread. Go to Google and search for NTFS-3G and download the free application and install it on your Mac. it will enable your Mac OSX operating system to read and write the files on the old PC formatted external hard drive.
You can keep working with this if you are going to link this external hard drive with its file system (NTFS) to a PC again. If not, it would be worth your while to sort the files on this drive, back them up to your Mac and reformat the external drive to Mac OSX Extended (Journaled) file system. It is faster, more secure, to name a few benefits. Then you can use it as a Time Machine backup, which is a clever and prudent thing to do.
It would serve you well to go on to Google and read up on the differences and pros and cons of the different file systems (NTFS, FAT 32, Mac OSX Extended) to prevent future frustrations.