Using old hard drive as external. do you need to install an OS?

hi everyone. i have what probably is a very simple question but it's something i haven't done before and i just can't seem to find the answer to my question on any of threads.

i have an old seagate 120gb sata1 hard drive that has been sitting around for months unused. recently i was thinking about wanting to use it as a "flash drive"; use it for back up and to transfer files between computers. i already found an external enclosure that i'll be getting. my question is for it to work, i mean to be recognized on different machines and to be able to back up and transfer files, do i need to reinstall win xp in the drive or would it work by just formatting it with fat32 or ntfs?

No, you don't need to install and OS. Just format it and you'll be good to go.

Assuming you don't want anything on this old drive, re-partition it (one volume containing all the capacity, NOT a bootable drive), then format as NTFS. That will ensure it is absolutely clean of old data junk. If you have software to do the backups work, etc, install it on this drive. Then you'll have it wherever you go.