I am having the same problem as Hoff. My IMAC desktop only shows blue screen. I didn't quite get what the solution was. Please help me...

This a software problem and the system may have got a virus in this is very rare as it is a mac and that macs dont get alot of viruses try updating the system next time there is a osx update or make a pointment at the genius bar at your local apple store

Unfortunately the previous poster has no idea what he's talking about, there are no viruses for OS X! Your blue screen can be cause by a variety of problem however a virus is not one of them! I would strongly recommend educating yourself on the subject, here are two very useful articles:

Viruses, Trojans and Malware - Prevention and removal: Apple Support Communities

Thomas Reed's Mac Malware Guide

If your profile is correct and your system is running 10.6.4, then restart the computer using the install media. If it runs OK then you know it's not something like a logic board.

However I suspect you aren't still on 10.6.4, please let us know the version of OS X installed.

I posted a possible solution for you here.

https://discussions.apple.com/thread/4063963?answerId=22693276022#22693276022