Unfortunately, Windows XP standby mode is so-so. It's normally not a recommended function, and on top of that Microsoft knows it has issues and has no plans to address it since it works a lot better on Vista and 7. Most stand by problems are caused by hardware drivers that are not compatible with the function. With a system that old there is a very big chance that there are no drivers that support the standby function. It's better overall to either leave the system on (disabling stand by all together) or just shut it down when not in use.