What do you want to do in the BIOS?? F2 should get you in the BIOS (according to the service manual).
F12 will start the boot manager. Just plug in your usb stick and start your pc pressing F12 immediately. You then should get the option to boot either from your hard drive, your optical drive or your USB drive. highlight USB and press enter. That should start the installation process.
Here is an cutout from the manual:
Changing Boot Sequence for the Current Boot
You can use this feature, for example, to tell the computer to boot from the CD drive so that you can run the Dell Diagnostics on the Drivers and Utilities media,
but you want the computer to boot from the hard drive when the diagnostic tests are complete. You can also use this feature to restart your computer to a
USB device such as a floppy drive, memory key, or CD-RW drive.
1. If you are booting to a USB device, connect the USB device to a USB connector.
2. Turn on (or restart) your computer.
3. When F2 = Setup, F12 = Boot Options appears on the screen, press <F12>.
If you wait too long and the operating system logo appears, continue to wait until you see the Microsoft Windows desktop. Then shut down your
computer and try again.
The Boot Device Menu appears, listing all available boot devices.
4. Use the up-or down-arrow keys to select the current boot device and press <Enter>.
For example, if you are booting to a USB memory key, highlight USB Flash Device and press <Enter>.