The timing before you can restart suggests a heat issue. As mentioned cleaning and checking your fans would be a good idea. Perhaps run with the case open to verify all fans are working.
Also view your Error Log via:
[1] Start Menu
[2] Help and Support
[3] Pick a Task
[4] Tools
[5] Advanced System Information
[6] View the Error Log
Also: look in your
Event viewer: rt. click
My Computer then click
Manage then
Event Viewer.
Or:
Administrative Tools/Event Viewer or
Start/Run/ eventvwr
Open the
system as well as the
application tabs & look for
X errors. Click error line for details.
Post back the
source &
event id &
Description especially of errors that seem to coincide with this issue.
Also check the
Information lines that might coincide with this issue.
How to view and manage event logs in Event Viewer in Windows XP