I have a Dell Dimension L550r that I have to reload xp onto.  I have the reinstallation disk but I cannot enter set up when the system reboots to change the boot up option from boot floppy first.  It keeps try to access the floppy and tells me that I have an invalid boot disk.  I need to know how to enter the set up to make changes to the boot options.  Normally on other systems you just press F2 during the boot and you will enter set up.

Does anyone know how to enter set up on my system?

Thank you.