|Why do you think you need to disable the F keys ? |
If this is on a laptop, malfunctioning laptop keyboards are common and are relatively cheap to replace with a new one. In the mean time, you can use a USB connected keyboard, or a PS/2 connected keyboard if your model has a PS/2 port, for most things.
If you want to press F8 while booting on a Windows system to get the boot choices menu and something else pops up first, you just cancel or Esc that and quickly press F8 again and you will get the boot choices menu.
On most computers, you DO NOT hold down the F8 key while booting - you repeatedly press it - but some laptops are exceptions to that rule.
Some keyboards have a special key or key combo that toggles the F keys off/on - it's toggled on by default, and may come on again after you reboot.
If the keyboard came with the computer, look for info about that in the Owner's or User's manual for the model. If it didn't come with the computer or if the info isn't in the manual, look up the keyboard manual on the Dell web site.
Other than that, I have no idea.