Sounds bad. Maybe you have an infection that has disabled your antivirus.
Try this one as an emergency backup: AVG Free Edition

Let's see, what else could it be… Oh yeah, your hardware might be acting up.
Try going into BIOS setup and reducing your speed, then scan for viruses
again to test it.

Another possibility is a disk error that hasn't been detected yet. That could
account for the problem with the Save As window, because it tries to build up
a tree of your folders as it opens up and an error in the structure can cause
parts of the OS to freeze up. Try scanning your hard disks for errors. If you
are using Windows, booting up in Safe Mode could be more reliable, though a bit
slower.