I got the same message when trying to use the context menu as standard user. UltraISO has been installed in the WinXP default program folder (C:\Programme\UltraISO, in my case).
Thanks to Ralph - his solution worked perfect.
For those experiencing the same problem, the following command line might be a faster solution than editing the registry manually:
CODE
reg add "HKCU\Software\EasyBoot Systems\UltraISO\5.0" /ve /d "%ProgramFiles%\UltraISO" /f
But of course the best solution would be an improved UltraISO installation 
Are there any plans to do this in one of the next UltraISO releases?
Just some additional remarks:
Used UltraISO v9.3.3.2685 / WinXP SP3.
I tried two different ways of installing UltraISO:
+ Under the standard user account via "run as /user:Admin" -> the context menu didn't work at both accounts, Admin and standard user.
+ Logged in as Admin -> the context menu worked at Admin account, but not at the standard user account.
After removing HKCU\Software\EasyBoot Systems in the registry, the context menu always worked at the current account. (Until starting UltraISO the next time. -> Registration info has to be entered again.)
Except for the mentioned (solvable) problem I'm very happy with UltraISO - thanks for this great tool!
Jan.