We had a Sony Vio (one of those "all-in-one" units) where the WiFi turned itself off (the WiFi had an external switch). The BT was also part of the WiFi and so it too was affected.
Rebooting would sometimes bring it back and during P.O.S.T. the WiFi LED would be on but then go out during the boot proccess (which suggested to me a driver or some other driver/service/app was turning it off).
Booting "Last known good" sometimes would restore it when a normal reboot wouldn't. But not always.
The times where rebooting did not fix the problem I would look for a somewhat recent restore point. 99% of time this always fixed the problem. For the times it did not fix it I would go into the Device Manager, uninstall the WiFi and BT devices, do a reboot and let the OS rediscover and reinstall the drivers.
We eventually retired this computer for other reasons but I never really did figure out what was causing the issue; this computer was really used to watch movies and do web browsing in the house.
It did not have any virus' or MalWare that I could find. Was always current with service packs and updates from Microsoft and Sony.
Rarely did we install anything on this computer. In fact it was perhaps the cleanest computer in the house :)
It could possibly have been hardware but as a test, I booted the computer and got into the BIOS setup screen. I ensured that the WiFi LED was ON and then left it that way for an entire weekend (close to 72 hours); the WiFi LED remained on the entire time. Rebooted the computer and the WiFi LED went out during the boot process as mentioned earlier.
I should also note that in the 3 years that computer was in service/use, the above would only happen 2-3 times a year. So it wasn't like I was battling this every day or week. :)
