The game freezes and doesn’t launch saved games.

When playing, the very first campaign can be completed up to the second level, but then, somewhere in the middle of it, everything starts to lag, and a blue, empty screen appears. Everything freezes, and a forced reboot is the only solution. Also, it's impossible to load saved games – the loading process starts, but then the game simply kicks you out in the middle of the loading bar (where it still says "Please wait, loading..."). No errors appear at this time. System: Pentium 4, 3.2; RAM 1000; 256MB X800. Why is this happening? Any suggestions would be greatly appreciated.