Windows 7 Black screen and other bizarre things happening.

Today when I was doing nothing really at all on my computer I decided to go on speedfan, after I clicked it on my desktop a black screen appeared which looked like my monitor wasn't plugged in (even though it was) and stayed like that for about 5 minutes before I manually shut it down. After I turned it back on, I was stuck on my desktop, with no desktop icons, the start menu icons were the default paper icons with the folded corner, my start menu button was still there, but I could not click it. My time in the corner was also messed up to something like 29/05/23957. So, I had to manually turn it off and back on again, and found that I could use my PC once more, however, some of my system tray icons were invisible, and my time was again randomized, however this time to sometime in 2009. Because I was able to use my computer, I simply restarted it and all was fine.

Simply, I'd like to know whats causing this and how/ or if I could stop it from happening again. Thanks.

Edit: After restarting I clicked speedfan once more and all is fine. I just don't know why this is happening randomly. This happened before but for no reason, my computer literally froze instead of displaying a black screen.