More likely as in the thread mentioned by Tesuo , the shutdown after the avast update that has corrupted the registry - Maybe fixed by running chkdsk /r from a command prompt as essexboy means.
But I'm not sure with a clean registry , it will not happen again with an other 2014 update.
Avast official words to reboot issue after update with xp:
We have finally found it. It was incompatability of unloading fss driver and some other our drivers. In about 4weeks we gonna release new version fixing also this problem.
However I can not say at 100% that the other points raised by essexboy are not true