Went to start up this iMac this morning and it would not past it's POST "Power on Self Test" replaced the Memory with 2 known...

First try turning up the brightness from the keyboard F2 and from the display system preference while looking at it from the external display. Then reset the SMC also known as the System Management Controller, and also zap the PRAM.. if that doesn't work your going to have too , troubleshoot the LCD panel and verify cable connections to the inverter and LCD. it's good that the External Display is showing that the laptop is functioning at least your taking the Logic board out of the mix.

Mr. Clean Magic Eraser's really do a wonderful job, We use it on all White MacBook's it brings back a nice , Brand New Appearance... DO NOT USE ON THE DISPLAY.. for the Display's a small bit of water on a micro fiber towel works very nicely..

Sounds like your computer can't find the system that it want's to startup ... Try your Install dvd and restart holding down the option Key. you should abe able to see your harddrive's operating system choose it and let it boot up completely.. Then go to System preference / Startup Disk and make sure that your System is Chosen.. Restart and remove the Install DVD...If your Install DVD is the only option you have to startup your computer.. you might have to look at a new hard drive or reformatting and reinstalling the system...

Yes there is...I imagine that you reset the SMC or System Management Controller by Shutting down and unplugging all of the Cables, including the power cord.. you need to wait at least 15 seconds, the reset occur's automatically once the Imac has been disconnected from the Power Source.. if this does not take care of the High Speed Fan Issue It sounds like the Tech did not plug in one of 6 Sensor's or one of them was not properly attached and came loose Power Supply ODD Temperature Ambient Temperature LCD Temperature GPU Temperature CPU Temperature All of these will cause the Fan's to run at Full Speed... If you feel like taking the Cover's and Display off, you could look around to see if one of them came unplugged from the Logicboard. You could carefully follow the take-apart and nose around a bit.. but be careful and Safe.

Don't know if you have fixed this Issue.. but if you havn't.. Go into your Home Directory/Library/Preferences and find the com.apple.universalaccess.plist Put it into the Trash and Restart your Computer.. Let me know if this fixes your Issue...

My other answer is incorrect because Boot Camp is 10.5. I guess I would use a Copy of Carbon Copy Cloner and clone my Drive to a firewire drive, then use the Disk Utility to partition the Drive and Clone it back onto my Laptop..

I would use Boot Camp to Partition Your Hard Drive...If your using 10.4 it should be in the Utilities folder that resides in your Application Folder.. As a suggestion.. I would update your 30 GB drive to something a bit more substantial.