Win 7 (and Vista) provides power user-definable power-management profiles, but there is no way through the GUI to set to the OS to "do nothing" when running on battery power and the machine reports battery power remaining at 5% or less ("critical" level). The choices are sleep, hibernate, or shut down. None of the above is not a choice.

Strangely, the command-line interface gives more control. From a command prompt with admin privileges,
powercfg -setdcvalueindex SCHEME_CURRENT SUB_BATTERY BATACTIONCRIT 0
will fix the critical battery level event response to 0, which corresponds to "do nothing".