How to reformat a time machine drive to use on a PC

My sister used to be a mac user and recently switched to a PC. She has her mac stuff backed up on two hard drives, one is formatted as a time machine. We've transferred all her important files to the other one that is not a time machine, so the time machine one is empty. She wants to use the time machine one as a backup drive for her PC but I"m not sure how to get it to reformat... help?