I use OO Defrag for many years now. Had it on win XP, win Vista and now run it under Win7 64 bit. Works fine. I let it run periodically every few month. Maybe because of that I never had the extremely slow reading of the runtimes that many here in the forum reported.
If you use the "complete name" method the first time, expect some hours of running time and a very heavy load on your hardisk.
OO Defrag installs a windows service that is active even if you don't use the software. With an older version of OO Defrag I had to set this service to "start manually" because it created some background load. The current version (v12) does not do this at least on my computer.

Together with the free ccleaner (www.piriform.com/ccleaner) that was mentioned in some threads (that tool that finds and deletes all the temp stuff that fills up the harddisk even the millions of hidden files Poser7 leaves in the Internet Explorer history folder and also fixes errors in the win registry) I was able to fix / avoid some of the problems reported here in the forum.