Go into XP (or Vista) and right-click on the "My Computer" icon. Left-click on Properties. You'll get the System Properties box on the screen. Click Advanced. Click Startup and Recovery Settings.
You'll see a checkbox next to "Time to display list of operating systems". Click the down arrow until you get past "0". It should go to "999" right after zero. That's 999 seconds of countdown time until your default OS will boot. Click OK, and exit the System Properties box.
That's over 16 minutes of time sitting on the screen counting down.
I don't know why you would need more time than that. That appears to be the max (999).
Then, anytime during this countdown time, you can choose your OS with the arrow keys and hit Enter.