Computer won't restart if turn off!

[megan]

Hi,
A big thanks to those who helped me with my synaptic packages manager
sources troubles, and yes I have been able to configure evolution after
I changed my email address.
A little more than one month ago I installed Ubuntu 6.10 on a HP
Pavilion a520n, from a dvd in dual booting with Windows XP. I don't know
if it did it the first time (I almost never shut off my computer)  or
later, but when I shut off my computer I couldn't turn it back on. There
was no more power. If I unplugged it and plugged it back a few minutes
later that worked the three first time and I had been able to turn it
on. After the third time it didn't turn on at all, no more power in the
computer. I thought I had a windows virus and I changed the hard drive.
The power came back. Then, I changed my computer of location and had to
change the power supply, it turned on ok. Now if I turned my computer
off again it does the same thing I have to unplug it to have the power
on. Does anybody knows why my computer is doing that? Did I configure
something wrong somewhere or is it a problem with the computer itself?
Your help would be appreciated
Thank you 
Megan