DELL XPS isn't a model number.

http://www.dell.com/support/article/us/en/19/sln284433/

Select your XPS

• XPS 18 1820
• XPS 200
• XPS 210
• XPS 27 7760
• XPS 400/Dimension 9150
• XPS 410
• XPS 420
• XPS 430
• XPS 600
• XPS 625
• XPS 630i
• XPS 700 Jet Black
• XPS 710
• XPS 710 H2C
• XPS 720
• XPS 720 H2C
• XPS 730
• XPS 730 H2C
• XPS 730x
• XPS 730x H2C
• XPS 8300
• XPS 8500
• XPS 8700
• XPS 8900
• XPS 8910
• XPS 8920
• XPS One 20
• XPS One 24
• XPS One 2710
• XPS One 2720
• XPS/Dimension XPS
• XPS/Dimension XPS Gen 2
• XPS/Dimension XPS Gen 3
• XPS/Dimension XPS Gen 4
• XPS/Dimension XPS Gen 5

You could look at the service manual for your XPS; there is usually a chapter call BIOS setup program but it will not help that much. The best option is to Google search the BIOS option you wish to know more about.

Should have mentioned I have a Dell XPS 8900 desktop

You can always post your questions about BIOS settings here and somebody should be able to help you answer them...