Hello, this is my first time in this forum. I've had exactly the same problem, and it must be something running in the background which doesn't when you are in safe mode. The finger of suspicion in my case points perhaps to Norton A/V.
You will have to disconnect from the internet and stop all operating software and especially any antivirus absolutly any programs that show on the bottom taskbar on the desktop. right click on a part of the screen and make sure that there is no power settings or screen savers actioned. any of the above will make the defrag program restart and its limited on how often it can do that before it stops altogether. The first run of a defrag can take a very long time depending on the size of the drives and how much information is on the drives, that will set the system up and do this at least once a month
I'm fascinated by the comments in this thread along the lines of "up to half an hour" and "can take a very long time".
Whilst I agree wholeheartedly that coming offline and disabling your AV is always sound advice when defragging and that size of HDD will have an effect, I'm gobsmacked at the lengths of time quoted to achieve a defrag.
I can only assume that these people are using an OS other than XP and/or their PCs are clogged up with all sorts of crap.
I use XP, always disable my AV and to defrag my 40GB HDD takes no more than 6 minutes on a bad day.