do you get a blue triangle come up? If so this could be active desktop recovery kicking in due to a software problem. Thius could be a virus scanner or corrupt foile spomewhere creating this problem. Maybe someone out there can give you a better answer but have a look at your virus software. If it's Mc Afee I am haviing a similar problem with this.
The other thing may b e a graphics driver causing this problem, so again check you are up to date on this.