Javaw.exe crashing

Question

We have a simple JNI call. We suspect that after the jni call returns something goes wrong and we get the windows XP crash dialog (telling us javaw.exe has crashed). Since we load and unload the DLL every time, we can keep using the program for a while but after that we get another message telling us some address cannot be read.
In the C code we have a printf just before we return 0 in the native method (the one defined in the .h file autogenerated .h with javah). So our native code returns correctly before hanging. Where is the crash happening?
The JVM does not crash, there is no error report. I'm clueless.

Answer 1

The only change is that java.exe crashes instead of javaw.exe.
I'm changing the C compiler. Maybe something changes if I use GCC.
The weirdest part is The java application and the native DLL (which is loaded and unloaded every time it is used through a wrapper DLL) can be used many more times after the first crash message and before windows it finally sais there was an error reading some memory address.
Something even weirder, if I use the app and close it normally before it crashes, when I start it again, it crashes at the first attempt to use it.