A couple of weeks ago, our Dell computer went blank. After going through all of the F8 menu options, I finally got the following error message,
"Windows could not start because the following file is
missing or corrupt: \WINDOWS\AppPatch\drvmain.sdb"

It follows: "You can repair the file by starting windows
setup using the original setup cd-rom. Select 'r' at the
first screen to start repair."

My problem is there is no first screen or "r" to select.
Although I do have a Windows XP startup disk, my built in drive doesn't work. I have a dvd and a cd connected by USB. But, the system doesn't recognize them.

How can I repair the file or by-pass the internal cd to get to the USB ports to load it from the disk?

Go to BIOS setup and select the boot option to USB device. I think on power on the computer you need to press F11 or F12 to go in to the BIOS. It show on the screen. Check if this works and let us know.

As "khakilang" states change the boot order to put usb first. Restart and boot from the usb-cd drive. Failing that a second hand ide cd rom drive will be pennies (2 cable install) IDE + power and off you go...