Latitude E6400 won't enter standby mode

My girlfriend has a new Latitude E6400 with WinXP SP3, Wake-on-lan disabled, fully updated, updated BIOS, only antivirus and Dell utilities load on startup. Whenever she selects standby or the laptop tries to enter standby per the power settings, it hangs indefinitely. I can hear the hard disk spinning every couple seconds but it never enters standby. Is there an application/BIOS setting/power setting/hardware issue that could be causing it to hang?