The file that is missing is the hardware information part of the registry. If you try to use the Last Known Good boot option it may boot but they usually don't. All that it should require is that you do an 'in place install' but it may be that all the registry files are corrupted and the in place install won't be able to find your installation. I have seen this many times and it must be some kind of undocumented feature of Xp and W2k that occasionally the the registry gets hosed or becomes unreadable.

Good news is that your data is just about always OK and if you have a copy of the system state data you can re-install Windows and import the system state data and woohoo! you are back in business.

Curios tip number 2 - back up your system state data
(number 1 was just back up your data using True Image)