Well lets see there is a bit of information missing that might help in diagnosis of problem, such as how big is the drive or drives involved?, is this the first time either program has been run? and which switches are being used? is this a direct install or a cloned/ghosted drive? and how are you actually determining that either program has actually stopped running.
The first suggestion is to run them from the Safe mode, especially if this is the first time this drive is being analyzed. This eliminates all the clutter of the interface and other interruptive drivers along with the power managment aspects.
Now with Scandisk - if the drive is big, as most all are now, and your doing the thorough scan it can really seem like it has stopped but hasn't - are the numbers incrementing? are there any indicators showing? If not, and your running from SAFE then you may want to check Microsofts Knowledge base as there are several know problems that Scandisk can run into that might be related to your particular hardware setup.
With Defrag - it can really depend on how much ram and freespace is available, whether there are some huge files, (it will look like it stopped but is really just trying to manage that massive file) and if you have the "reorganize" the drive switch on of advanced features - which I never recommend, that one can be a real killer and I have seen it run for DAYS trying to organize things.
Those are what I would look at - but then completely redoing a drive never is a bad idea and makes stuff nice and neat.
Hope that helps you.
