Random sleep while working without reason despite power settings always ON hibernation disabled.

[giar2]

For the last couple of weeks the computer started to go into sleep or hibernation mode, at random times, without warning message and no reason as the power settings have hibernation disabled and all other power options, including sleeping are set to "never", or always ON. Also it happens either 5 min after booting or an hour after, even when using applications and having open windows. The only way to get back to using the computer is to do a forced shut down from the power button and then reboot.

Computer has been working fine for years. I have Windows update running daily, updated antivirus and no registry issues. This is getting very annoying, please help with solutions or suggestions. Many thanks for your time.

[giar2]

In Case it may help, here are some specifications:
HP Media Center PC m7250n
PentiumD 820 (S) DC 2.8 GHz:
800 MHz front side bus
Socket 775

Chipset: Intel 945P

Motherboard: Asus P5LP-LE
HP/Compaq motherboard name: Lithium-UL8E

Hard drive: 250 GB SATA 7200 rpm

Memory installed: 3GB DDR2 SDRAM

Video graphics: ATI Radeon x300se
128 MB DDR memory. I/O Ports: Composite, S-Video, VGA

Operating System
Microsoft Windows XP Media Center 2005 with Update Rollup 2
Microsoft Service Pack 3

Power Supply: HP-D6057F3R 300 W.