'''Try to boot from a different source/volume.''' I think you have damaged OS files on your boot drive. ''A successful boot from a different source would confirm my diagnosis.''
You could '''try a Safe boot '''if you're running OS X. (Shift boot) That may bypass the corrupted files and if it does tossing out preferences ''(you'll have to reset all your application preferences).'' may fix the problem. Other files not part of a safe boot would be some fonts, and extensions.
On a machine that age''' a PRAM reset''' may also work. Hold down at power on command-option-P-R through three (3) restart chimes then let up.
''If this answer is acceptable '''please remember to return and mark it'''.''