The MobiPocket Reader software is still about as good as it gets under Windows, but it is never going to be updated and MobiPocket is essentially dead as a brand.
So, I suggest you strip the DRM (encryption) from MOBI ebooks and switch to another reader app. Two that I like and which will read DRM-free MOBI directly are Kindle for PC and FBReader. If you use Calibre to format shift to ePub there are many additional options, see
Reading Adobe ePub on the Desktop and there are also DRM-free only options like Calibre's own built in reader and FBReader again.
See Apprentice Alf's Blog for more on DRM.