Re: Computer on and off
I really feel this is more of a hardware problem as opposed to a Windows software problem but you could run sfc /scannow, this replaces corrupt system files in XP if you are concerned it is Windows related.
Place XP cd in the rom drive
Cancel auto run
Start > Run > Type in sfc /scannow (note to space between sfc and the /, if typed incorrectly it will not run)
My thoughts on the hardware run to bad psu or bad ram (more than likely ram) you could try running one stick at a time to verify if it is the ram and put a volt meter across your 12v to check for correct voltage. Another possible cause could be heat related, have you checked your cpu heatsink for dust build up which would be impeding air flow / cooling??
If you're not happy to try these options yourself then I suggest that yes you do take it to a shop.
As to formatting it, they should be able to do a repair install of Windows which will leave all your data intact.