Am trying to revive my old desktop from the "coma" it is in. Each time I have tried to boot up, I get message "Windows could not start because the following file is missing or corrupt: \WINDOWS\SYSTEM32\CONFIG\SYSTEM" (message continues) "You can attempt to repair this file by starting Windows Setup using the original Setup CD-ROM"
Will not boot from original Windows XP Home Edition (with Service Pack 1) CD.
I found UBCD511 on the internet, downloaded ubcd511.ISO onto my Vista based desktop, and burnt it to a Datasafe CD-R at 16x speed. Received confirmation from Express Burn software that ISO file was properly burnt.
However, on trying to boot old desktop I got the Ultimate Boot CD V5.1.1 menu screen, with BIOS highlighted, but am unable to access any of the listed menu items.
Any help or advice that improves on the present situation, particularly advice that enables revival of Windows will be much appreciated.

If you can describe the "problem" to boot the Windows XP CD, maybe someone can give you some tip or direction. There are many several possible reasons for it, from a bad CD, the optical drive itself, BIOS problems, power (battery alone against AC power), HDD problems...

Have you tried testing the "bootability" of the XP CD using other computer / optical drive? Can you try a USB drive with XP install / repair?

About UBCD, start here:

Official download: http://www.ultimatebootcd.com/download.html

Check if the download is correct: http://wiki.ultimatebootcd.com/index.php?title=Tutorials:Calculating_Hash_Sum

Some tips to correctly burn the optical media: http://wiki.ultimatebootcd.com/index.php?title=Tutorials:Burning_UBCD_to_Optical_Disc

To burn the optical media, try CD-RW or CD+RW (re-write CD, not just +R -R, and not DVD) using 4x or 8x (and not less nor more speed).

When you say you cannot access any other menu of UBCD, what do you mean? The cursor keys won't work? Try changing the "Fn" key, and/or the "num lock" key, and trying the "normal" cursor keys and the "numerical pad" cursor keys. An alternative would be to try an external keyboard.