A power out?
Do you have a desktop PC or a laptop?
Because if you have a laptop, in this case, the first thing to try is to take out the battery from it and try to run it only on power ...
If not, and you still want to recover things and get in your Windows you could try the Recovery console. But you need a win xp CD.
You could do a chkdsk, bootfix, or disable the welcome screen even (in some cases, but that's a little tricky) or even make a clean new user to see if it opens...(but you need to know a little cmd, or follow step by step instructions...)
If that doesn't work ,you can go with the repair install...