XP Home Woes

My folk's computer (Dell PIII 600) is acting up.

You can start it up, get to the XP login screen, log in.
The desktop appears, applications start to load ...

The machine reboots itself.
On the reboot it gripes and does a checkup of the hard drive(s), correcting errors as it goes.

I have not had time to do a safe mode boot, nor a restore.
Those will have to wait until Saturday.

Any Suggestions?