That Pentium D will shut down in 15 secs, if the cooling is not sufficient. If nothing has ever been done to the board, get a new CPU heatsink and fan, install it and try again. I use Arctic Cooling Freezer 7 Pro
on my 775s. Since it doesn't sound like your mom is overclocking, you could find a cheaper one. Make sure the "thermal grease" is present on the new heatsink - if not, you'll need some to provide adequate heat transfer from the cpu to the heatsink.
If you replace the HSF, and still no start, you may have to get another CPU.