Does it happen only within the os( like the winxp time gets reset) or also in the BIOS gets reset?
Have a read here: http://www.computerhope.com/issues/ch000386.htm
See if disabling Cool n' Quiet from BIOS helps.
It could be software related, though very unlikely, but technically a program (malware?) can get to reset or manipulate BIOS settings, so try doing a clean install as last hope