Hold down the keys immediately after you press the power button.
Have you had any problems with the keyboard in normal functions? For instance, do the P and R keys skip? Do the command and options keys ever skip? You can check this by opening the keyboard palette and pressing the keys a few times to see if they register in the palette. The keyboard palette should be available in the international keyboard menu once it's enabled in the "international" system preferences".
As silly as it is, but can happen - it happened to me, so I thought of sharing; instead of option (alt) I insisted on cmd
ctrl+pr, which of course gave no results.
It had taken a few rather nervous minutes before I realized that I needed opt (alt) and not ctrl.
I am having major issues. I've done a PRAM reset and am now trying to reinstall Tiger on my iMac (I made the mistake of updating to 10.4.11 this evening). Now, whenever I reboot with the DVD inside the drive, it pops out after I release the C key. Is there something else I can do? Otherwise I have a 1200 dollar computer that doesn't seem to want to get past the gray bootup screen!