If it were lsass.exe it would be a system file, but Isass.exe is malware:
http://www.softwarepatch.com/tips/isass.html
Possibly other malware is keeping it from doing its insidious task, or
maybe it wasn't put together properly by its loathsome maker.

The extent of my sagacity is to google the program. What people
do with contaminated computers is too multifaceted for me to follow,
kind of like choice of religion and denominational schisms. If, "Get a guy
to clean it up," is doable for you, that's a major time saver. Otherwise,
buying a new computer or a clean installation of Windows fixes it. Try
booting into safe mode and rolling back to a restore point if you can't
or won't do the more drastic things, then run infestation clean up after
you are able to log on. Hopefully a regular in this group (like PA Bear)
will respond, and you'll be patient about following his advice.