whats a java webstart directory
Dunno?, why are U asking perhaps you misread something?
Java Webstart is a way to distribute your Java app over the web. Dunno what you mean by java webstart directory 
Unless you’ve been asked to clear your WebStart cache directory?
This changes depending on operating system and Java version, but on a Windows XP box you could do worse than looking here:
C:\Documents and Settings\USERNAME\Application Data\Sun\Java\Deployment\javaws\cache