Startsup, apologizes, switches off, repeats

Hello,
Yep, this has probably been mentioned in other threads, however, this is my problem:
When i turn on my asus pc running xp i get the 'sorry for the inconvenience' b/w screen with startup options, i start it normally, i get the windows xp logo then seconds later the machine switches off and it reboots. Ive tried stating in all modes but the same thing happens. The day before I had been trying to fix my broken external hd, while it was plugged into my computer, so perhaps i had shorted something? but the computer did go the next day for a couple of hours. But now this is happening. So, my question, whats the likelyhood of recovering my data, and any ways i could remedy this myself?