Wow you would need to be a little more specific than that.
XP home or pro?
Does it go through the shutdown routine but not power off?
Do either of you know much about pc's?
It is the same OS with some networking stuff disabled, so no diff.
If windows starts the shutdown procedure but never actually turns off, I had a similar issue.
Firstly, do a windows update!
Make sure he has the latest drivers installed for his hardware. It should be available on the Acer website. He can also go download directly from the manufacturer. A good utility to check what the hardware is, is SIW (use it to identify the manufacturer and device). Look specifically for any ACPI, BIOS or motherboard related driver updates.
If windows does not start the shutdown procedure, try using an alternative method:
Start | Run | type "shutdown -i" | OK