How accurate do you want it to be?
It's probably a lot easier to simply monitor AC current rather than trying to measure actual power (taking into account both voltage and current as well as the phase relationship between them).
If you just want to measure current, then you can get a PC-interface multimeter (either USB or RS-232) with an AC current probe (that you can clamp around one conductor of an AC power line) and sample it with simple data collection software. You wouldn't have to do any analog-to-digital conversion, rather that would be handled by the multimeter.
I use a Kill-A-Watt meter for doing such measurements (actually I have two different models that I use), but that meter does not have an output for use with an automated PC data collection system. It is possible that a place such as smarthome.com has power monitors with PC interfaces, but I have no experience with any such units.