No admin access to BIOS to change boot priority

Enter your BIOS setup utility and change the boot sequence so that the CD is the first entry in the list of boot devices. The Ubuntu minimal CD being very small (<40MB) and text only should be able to boot successfully, even on older systems where the graphical Ubuntu live DVD/USB does not boot successfully. There is no need to use a different Xubuntu CD, because you can select and install the Xubuntu desktop from the Software selection menu inside the Ubuntu minimal CD installer.

You should be able to enter the BIOS setup utility without administrative rights, unless it is password protected by an unknown password, and even so you can remove the BIOS password by removing the CMOS battery from the motherboard for 5 minutes and then putting it back in. This will also reset all BIOS/UEFI settings back to their defaults.

^{CMOS battery}