I downloaded and ran version 9.138 to an XP SP3 home machine.
I did an analysis, then started the defrag. When I returned a few minutes later, the screen was garbled with graphics. I pressed CTRL/ALT/DELETE once to see if task manager would pop up. It did not. Then, I tried ALT Tab to try to switch to another application ( Solitaire ), and nothing apparent happened.
Within a minute, the machine tried to reboot itself.
I got the message "Missing operating system".
Many magic words later, I booted from the original CD, and got my desktop back.
I then restarted the system, and it appears to be the same as it was, pre Defraggler.
If you have any ideas, or need additional materials like an HJT log, let me know.