Suggest you to get a new HDD for backup because your OLD HDD may fail at any time & lost all datas on it.
However, to do what you want, move your important files on the old HDD to another backup device or the Macbook Pro temporarily then put it in a either a caddy or with suitable connecting cables & connect it to the Macbook Pro.(Make sure you check & can open those files after moved)
In Disk Utility , select Partition tab (if you don't see Partition tab then you've to erase the drive. Partitioning an external hard drive for Mac and PC (follow Kappy's advice) or How to partition a startup drive (click 1-5 steps on the left side lists).
The partition you wish to use on a PC you should format MSDOS (FAT) or ExFAT (if supported on your PC) but if you prefer using NTFS then format it with a PC.