I presume you mean a natural re-boot? (as in, "it shuts down naturally, then re-boots" rather than the screen going black then re-booting). This is a standard annoyance of XP and (quite probably) nothing to do with power, but is more likely some form of system failure (this is the standard procedure for XP when encountering a failure) - you will need to check the event log for any messages showing hardware/system faults - Start, Settings, Control Panel, Administrative Tools, Event Log. You can if you wish, disable the automatic re-booting - see
http://www.aumha.org/a/shtdwnxp.htm
for more details