I had this happen to me right after installing VirtualCloneDrive. So I went back to a restore point and it has been fine for a week.
Monitors: Dell 1800FP 18.1 LCD, Sony Bravia 40" HDTV
I have the same issue on both of my windows 7 media centers. One is running a pvr-500 with hdhomerun and the other is running a pvr-150 with hdhomerun. There is no way of predicting when this error will occur and sometimes it will randomly fix itself after I shutoff the media center for a day. I have been able to go a couple of weeks without this error, but when I am scheduling recordings, that is when it seems to occur most.
This problem is driving my crazy. Otherwise I am loving media center 7 and can't wait to throw vista in the garbage.
Not sure if you are still interested in this, but i had a similar problem and found a solution. Check out my post here.
It turned out to be a permission error.
GF