I have a system with sli with two 7800gtx 512. The boot screen won't show with any monitor, the monitors turn on only when windows starts. Moreover if I open a command prompt and make it full screen the monitor switches off (meaning it gets no signal) and back on if i close it with alt-f4. This wasn't happening in the first few days, it started happening with no apparent reason. What can it be? Everything seems to work fine apart from this.