in some sleep mode, pc can wake up by network device,so when you try to access file, it wakes up; this needs both hardware and software support, if you have win7 most likely your pc has this capability. if you have XP, then probably hardware is too old to have such capability, and same to the software.
I believe you can use powercfg tool to see which device can wakeup your PC.
not an expert, just try to share what I know--
