You have not specified which operating system you are running, but for Win98, for example, the program will probably have installed itself in c:\program files\UltraVNC. Inside that directory there is an Uninstall Application called "unins000.exe". Run that first. You could probably manually delete any left-over files in that directory too.
Now for the register. Start>Run> type "Regedit". Export the register first to somewhere safe just in case. Run Registry>Export Registry File. Then run Edit>Find and type "vnc". Any time it finds an entry, just hit the "delete" button. Press F3 to find the next entry etc... Once finished, Exit from the Registry Editor. Reboot.
This is a very "down & dirty" method that would probably not be recommended by any expert. I am not an expert, just an average hack, so no responsibility is taken! Good Luck.