Usually, one can remove JBuilder from "Add/Remove Programs". If the installation was INCOMPLETE, one probably cannot find the correspoding entry in "Add/Remove Programs". (But I suggest you check again to see there is an entry.)
If you really cannot find the entry in
"Add/Remove Programs", you may open Windows registry, go to HKEY_LOCAL_MACHINE|SOFTWARE, delete the entry related to Borland and INprise, and then go to Windows Explorer to
delete C:\JBuilder.... Next Reboot the machine, re-install again.