yesterday I started defragging, with about 104 GB free space on my harddrive left. I just woke up, looked at my pc, and saw this has come down to just 40 GB. Where did that go??? Lesser space = slower defrag, so it's taking longer and longer to defrag.
System Restore Points, and, if you're using Vista/Win7, Shadow Copies, maybe ? They're all in C:\System Volume Information\.
Use something like TreeSize Free to determine which folders/directories are using lots of disk space, before running Defraggler, and after...