Can someone help me figure out what is going on? [View all]

[Paper Roses]

I have an old HP computer, HP Pavilion 763n running XP SP3.
Most things seem to work fine but a problem of 6 months ago has resurfaced. Before it gets any worse:

When I boot up in the AM, my computer shuts down after a few seconds. This happened once before and the computer guru I took it to said he cleaned it out. ( of what, I don't know). It is an old machine and he did not want to see it again. He added memory. I have plenty of memory available.
I don't use the machine for much of anything but Internet and an occasionally Microsoft Word.

I try and keep the system clean of old junk, clean out Temp files and the like.

The computer guy told me the machine is too old and he no longer wants to work on it.

I am in no position to buy a new machine. I was given a Laptop w/IE7 and do not like it. I like this old timer.
I am afraid this current machine will keep getting more quirky.

What could cause it to shut down after 3 to 9 seconds. Nothing else seems to be wrong at the moment.
Any ideas?

I have a feeling that the computer repair guy just wants to sell me a new machine. I know lots of folks who have machines older than this one. I just want to find out what would cause this shutdown and hot to correct it.

Nothing else seems to be wrong at the moment.
What do you think?