I ran into the same issue after i upgraded to Win 8.1 on my HTPC. My HTPC does not have a TV Card, but i had the same SYSTEM event preventing sleep.
Open a command prompt and execute the following command:
Powercfg -requestsoverride Driver "Legacy Kernel Caller" System
That should fix it.
Thread prefix updated
Open a command prompt and execute the following command:
Powercfg -requestsoverride Driver "Legacy Kernel Caller" System
That should fix it.
Thread prefix updated