I have installed over 100 NV500 systems and I don't have that problem. Check your hardware make sure you are using Windows XP Pro with at lease 512MB RAM and Intel chipset Motherboard. Make sure Windows is up to date and you are using 7.1 or higer DVR software. I recommend 7.5.
also, I would recomend installing a battery backup, and configuring the software for auto-reboot, auto record, and in the bios, to startup on ac restore.
like was mentioned above, if the system is not secured(system password, and lock box), this would also be a good idea.