Question: With 1.3.1 we got 4GB heaps on Solaris, why can't I get this to work on Windows?

Answer: It's due to fragmentation of the address space. After Windows has loaded all its stuff, plus the java dlls, the largest available continuous chunk of addresses for the heap is about 1.4-1.6G on Win/NT. Might be smaller on Win/98.

If you really need more space they can try rebasing java.exe and the JDK dlls higher. But that won't buy you much (maybe a couple hundred meg).
from the Java Hotspot Virtual Machine FAQ:
http://java.sun.com/docs/hotspot/PerformanceFAQ.html
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