Could not reserve enough space for object heap

JVM can not reserve more than 796M

for example -Xms797m -Xmx797m
got an error:

Error occurred during initialization of VM
Could not reserve enough space for object heap

for example -Xms796m -Xmx796m - OK

Sever: MS windows server 2003 Service Pack 1

java version "1.4.2_04"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2_04-b05)
Java HotSpot(TM) Client VM (build 1.4.2_04-b05, mixed mode)

update : not 512 but 796m