Lost Free Space

I have just downloaded Defraggler and have run it for the first time. Before defragmenting I had 76.8GB free space on my C drive. After defragmenting the free space has reduced to 21GB! How has Defraggler used so much free space, and how can I get it back?

My OS is Vista Home Premium.

Urgent help would be appreciated.

Thanks and regards,

royw

What's the total size of the drive ?