I just downloaded and ran ubcd 503, and tried the Avira Antivirus.
What do you exactly mean by "ran" ubcd 5.0.3?
Before scanning, I tried the "Update" function, but it didn't work.
Do you mean before customizing?
Has anyone else been able to update the virus definitions successfully using 5.0.3? Or am I missing something?
My guess is that you burned the iso image and then tried to update. It's just a guess, so I may be wrong.
The procedure is:
1_ Download ubcd503.iso
2_ Expand the content of the iso image
3_ Run one by one the scripts to update the antivirus definitions
4_ Run the ubcd2iso script to obtain a new updated iso image, or the ubcd2usb to obtain ubcd on usb stick.
Optionally, between steps 3 and 4 above, you could customize anything else you want according to the relevant instructions.
Finally, if you get an updated ubcd iso image by following the above steps, then you can appropriately burn it to an optical media.