On my system, JSTL 1.1 library contains the jars:
jstl.jar
standard.jar
As to where they are located, they're buried in Netbeans.
Specifically, (on my machine, a Mac) there is a .netbeans folder in my home folder.
Pretty sure this exists on Windows as well.
Within .netbeans, there's a folder for your specific version (6.5 in this case).
Within that, is a modules directory.
The jars are located at:
.netbeans/6.5/modules/ext/jstl.jar
.netbeans/6.5/modules/ext/standard.jar
The association of the jars to the library are made in a file:
.netbeans/6.5/modules/org-netbeans-modules-web-jstl11.jar
If these files are all intact, you might want to try and remove the folder:
.netbeans/6.5/var/cache
Netbeans creates this, and if Netbeans start going stupid on you, erasing it and letting it rebuild it may clean things up for you.