Most of the times when the system gets halted improperly due to power outage or due to some other reason, the file system gets corrupted and Windows is unable to mount the boot drive properly. There are other factors that also result in unmountable boot volume. These include faster UDMA modes configured in BIOS and using 40 wire data cable for the hard drive.
Load the fail-safe defaults in your BIOS and use 80 wire data cable for the hard drive for optimum performance.
If the file system is demaged (which usually is the case)