Software Update Gets Stuck While Installing

When updating Apple software, the software downloads and the computer restarts, but the computer gets stuck at the far end of the installation bar. How do I fix this?

Computer--27" iMac, model11,1

Processor--Intel core i5

Speed--2.66GHz

Operating system--Mac OS X 10.6.8

Storage Capacity--1 TB

Memory--Bank 0/DIMM0:

Size: 2GB

Bank 1/DIMM0:

Size: 2GB

Bank 0/DIMM0:

Size: Empty

Bank 1/DIMM0:

Size: Empty