Windows Vista Home Premium SP2
2x 250gb HD's in RAID 0 - partitioned three times (recovery drive, Vista OS, XP OS)
I'm running a dual boot system with XP as well and am only having this issue while using Vista. Leads me to believe that it is not hardware related.
When I am actively using my PC everything is fine. No out of the norm issues. If I leave it idle and do something like watch a movie or just leave it unattended then it will go to a blank black screen with a movable mouse after about fifteen or twenty minutes. One occasion it has went to my login screen and after logging in it was just a blank black screen. Another time it left my taskbar up and I was able to actively use open programs but could not open new ones or the task manager. Attempting to open the process viewer in Comodo froze the program.
I have read many things about a black screen with a movable cursor on startup but nothing about it happening from an idle state and working just fine while in use.
I have changed my power settings so that it wont sleep, hibernate, screensaver, etc. No change.
Ran the repair option with the Vista CD and it came up fine. Memory test was also clean.
Only time something goes wrong is when it is left idle.