Just downloaded and ran latest Defraggler version which is the first version I could get to work on my XP machine. (Old versions wouldn't go past file list generation)
When I rebooted machine it went into a boot-up endless loop. Selecting safe mode or anything else also ended up in the endless boot-up. Selecting windows recovery console for the purpose of running CHDSK gave me a blue screen of death which recommended disabling drivers, defragmenters and anti-virus software. I did select 'replace windows defragmenter' when I installed new version. Also the boot-up defraggler had been working fine for a long time even though normal defraggler did not work until this version. I started out with a quick defrag, restarted and then ran the normal one just before this error occurred.