With my new 901 running stock Xandros and no BIOS update I was completely unable to access the BIOS during start-up. No amount of pressing the F2 key whether before, during or after the start-up screen would ever take me to the BIOS, and I tried many, many times. I had the same problem trying to access the boot device menu when pressing the ESC key i.e. that didn't work either.

The solution to this, as pointed out by prx (and probably others)

http://forum.eeeuser.com/viewtopic.php?id=35381 

is to set the BIOS>BOOT SETTINGS CONFIGURATION>BOOT BOOSTER parameter to DISABLED. This results in the "Press F2 to run set-up" message being displayed at the bottom of the start-up screen.  A single press of F2 will then take you to the BIOS.

The problem is, how to get to the BIOS to disable Boot Booster? This is how I succeeded.

1.    Boot the eee normally (into easy or advanced mode, it doesn't matter)
2.    Press AND HOLD the power button until the eee switches off
3.     Now press the power button again to restart the eee
4.     For some reason, this now ensures the "Press F2 to run set-up" message appears at the bottom of the start-up screen and so the BIOS can be accessed easily
5.    When you get to the start-up screen , press F2 to enter the BIOS.
6.    Disable BOOT BOOSTER and F10 to exit and save

This also makes it easy to get to the Boot Device menu by pressing ESC.

I hope this is of help to other novices getting going with their eee's