A few things I would try first to see if it helps....
1. Re-install defraggler, reboot, and then try the uninstall again using defragglers own uninstaller. If no luck then....
No error then, but then again I didn't use Defraggler when doing this method, so clearly it's obvious no error will show up.
2. Run a system file check (SFC) see below for info on how to do this
http://support.microsoft.com/kb/929833
Did this anyway and the scan came out clean with no errors.
3. Make sure Windows is up to date.
Everything is up-to-date.
4. Make sure all virus/malware programs are up to date and scans are clean.
Couple of days ago I scanned my pc with Avira which came out clean. Also scanned with MBAM (no detections), and to top it of I also scanned my pc with Adwcleaner and nothing came up.
But yeah, I can safely say that the problem clearly lies within defraggler since I also tried it on a different PC.
If I was you, you should just test it out the way I told it in my first post, so we can be sure about it that probably everyone gets the same error message.