What hang-9 said, probably either some pirated win copy, or possibly more likely, your computer turned off windows aero due to high resource consumption, or you messed with the game launcher and told it to turn off windows features unknowingly. To check if you still have ultimate, click start, right click computer, click properties, it will tell you your os version there. While you're at it, scroll to the bottom of the window to see your activation status.
There's pretty much no way for your OS to automatically downgrade, and windows 7 basic is a 32 bit os, I'm going to assume you were on ultimate 64 bit. What you think has happened is actually impossible. Just a windows scheme change, right click desktop and click personalize, and it's probably on windows 7 basic there, change it to 'Windows 7' under the aero category.