Same for me, portable or installed RegBackup 3.5.0.3:
I cannot open the program by clicking the desktop icon or the *.exe file.
I can see the program running in the Task Manager, but no windows is open.
More of that, the program is permanently using 25% of CPU...
This problem occurs with a Portable running Windows XP SP3.
And it runs ok with another Portable running Windows XP SP3. Very strange isn't it ?