Yes, if the file was taking up that much scratch, then you need more than 100gb free on your hard drive or use a second empty hard drive for the photoshop scratch space.
Most likely whatever you were attempting to do would have required more then the 99GB that you had free.
Sometimes people forget that resolution values can result in huge images, or that loading and combining many images can require a lot of scratch space.
Thanks but now that PS is uninstalled my harddrive still only has 16GB free whereas it did have 99GB. I've now put PS on a mac so want to return my pc harddrive to how it was or delete whatever is taking up the space. Any ideas?
Uninstalling the app wouldn't take space away from the drive, it would free up more space.
Check the TEMP folder that R_Kelly pointed you to.
Recovery files or leftover scratch files from a crash are all that I can think of that Photoshop could leave behind.
Use the Disk Cleanup facility of Windows, if that's what you've got. That can clean out your TEMP files if you instruct it to.