Problem with logging off

I have searched the threads, but could not come up with anything.

My problem is random, sometimes when I log off, my iMac won't go to the Log In screen. It will show my background, but nothing else.
No menu bar, no dock, nothing.

I have Leopard installed. The current version 10.5.2.

I don't remember this happening before I installed Leopard, but I don't know that for sure.

I have to hold down the Start Up button on the back to restart the iMac when this happens.

I have a printer and two External HD's attached, that's it.

Thanks in advance.

Model Name: iMac
Model Identifier: iMac5,1
Processor Name: Intel Core 2 Duo
Processor Speed: 2 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 4 MB
Memory: 1 GB
Bus Speed: 667 MHz

I'm usually just running Safari (I quit Safari before logging off.)
I did a regular upgrade. I'll try disconnecting my peripherals before logging off next time.

Thanks.