HD not reclaiming free space

I have a user who had 70 mgs of space left on the C: drive. He deleted a folder that was 300mgs(temp internet files), but the C: still shows only 70mgs left. The folder is empty(including hidden files) and the properties of the folder shows 0 bytes.I have checked the pc for viruses and checked the disk for errors. This is WinNT.
Any ideas how to force the HD to reclaim the free space?
Thanks.