When I try to run JBuilder I get the error:

Internal Error, failed to start JVM - unable to create temporary file

What can I do?

This is a problem being detected by JavaLauncher.dll when it tries to create a temporary file on the Windows platform.

That particular error occurs when it is trying to create an initial zero-length temporary file, so disk space isn't likely to be the issue. Either your temporary path points to a directory that doesn't exist, or you don't have permission to write there. According to the Win32 API documentation, the directory being used will be defined by the TMP environment variable, or if that isn't defined, the TEMP environment variable, or failing that the current working directory.

If none of the above seems to lead to a solution, commenting out the "exportenv" line in jbuilder.config file will eliminate the problem at the expense of some functionality when launching external processes like CVS.