Strange defragmentation tool : free space reduced after defragmentation

"As a side note: It's always advisable to do chkdsk before defragmenting.

You'll need to find out where the space has gone.
Things to try:

Right-click the C drive in Explorer and choose Properties. Verify that the total used+unused space still corresponds to the disk size.
If the total disk space is correct, but the used part is enormous, find out where the space has gone to. Using a visual tool such as Scanner, you can find that out easily.
EDIT: Run CCleaner to remove unused files from your system."

"2.6 GB free space after installation is not a value Windows will be happy with for long. If you can, you should reinstall on a bigger partition. The unavoidable patches will fill that space up in no time, and Windows will be in trouble.

Perhaps the Swap file is set to ""automatic"", and during the defragmentation Windows had to allocate some virtual memory.

Check your event logs for disk errors."