running the live cd

[MICHAEL WEAVER <michaelweaver1 btinternet com>]

I have been trying to run the live CD of Ubuntu.
I put the CD in the drive, wait while the drive churns around, I keep 
waiting and then it just goes into Ubuntu.
When am I supposed to press F5 to chose the accessability?
Am I supposed to wait until it goes into Ubuntu itself when I hear the 
startup sound or am I supposed to just wait a few seconds and press F5 
before it goes into Ubuntu before I press the down arrow 3 times?
I want to see if I can get Ubuntu with Gnopernicus working as a Live 
version on my desktop before I install it but I have no Sight at all and 
nobody to help me as I live alone.
At the moment my desktop runs only Windows XP.