"Just had to replace power source" - because the original failed ?
Several years apart, I've twice had PSU failure. After replacement, on both
occasions found that the original failure had caused further damage, once to
a motherboard, twice to a floppy drive and once to a hdd.
Unfortunately, PSU's rarely cut out completely (like a blown light bulb) -
the failing/problem circuits tend to produce excess voltage prior to blowing
(or just continue pumping out excess voltage) with catastrophic effects on
pc hardware, or unpredictable effects. There are many different output
lines and voltages, each supplying different hardware or hardware elements.
Your description of XP shutting down immediately after start-up is similar
to one of my "failure" episodes (bad sectors on boot area of hdd). This
could also be an independant cause of your problem - run "chkdsk" to see.
In both of my instances, mea culpa. I had installed additional hardware
over time - additional RAM, sound card, second hard drive and second DVD
drive, but forgot to check the PSU ratings - easily done. Worked for a
while, then ZAP!
Suggest you check over your individual components for integrity. I hope for
your sake the damage isn't too great (and, if you haven't already, back up
your data soonest - my hdd worked spasmodically for about 8 boots before
going terminal).
