Unknown USB Device ( Device Descriptior Request Failed )

I think the main problems of a PSU are +5VSB and -12V Lines, mostly this lines have not enough power.

CountMike said:

psu would have to be grossly under-powered to be felt thru such small power using devices like usb and even than it would hinder other parts first.

i am not sure about this.
try to connect a external hdd that consumes a lot of power to a usb 2.0 port and it immediately goes bad. Happened to me with 2 HDD.

The bios shows normal voltage, I just checked that. I have an external usb hdd always connected, it works fine. for years. no problems with it. i don't get blue screens, computer does not freeze, games work fine, work works fine. the psu is 4 years old, Seasonic are good PSUs as far as I know.

Rostos said:

the psu is 4 years old, Seasonic are good PSUs as far as I know.

I am not saying that Seasonic are "bad" not at all, just may be too small, I would prefer to have a 650W or even 750W PSU.

Voltage is not the Problem but the Amper. And will be very difficult, by watching the Voltage, to see that the PSU has note enough Amper.
If you see a significant Drop in Voltage because of too less Amper, then it will be near to Burn in Fire.
Look at MB Manual and see the Amper needs. See also the Amper needs on the 6-Wire Plug of the GTX 660. Add all the Ampre together and see if PSU has that amount.
Why should your USB not be working? Something must be wrong. If it is not PSU, then a bad MB will be the question.

Adalwar said:

I am not saying that Seasonic are "bad" not at all, just may be too small, I would prefer to have a 650W or even 750W PSU.

Voltage is not the Problem but the Amper. And will be very difficult, by watching the Voltage, to see that the PSU has note enough Amper.
If you see a significant Drop in Voltage because of too less Amper, then it will be near to Burn in Fire.
Look at MB Manual and see the Amper needs. See also the Amper needs on the 6-Wire Plug of the GTX 660. Add all the Ampre together and see if PSU has that amount.
Why should your USB not be working? Something must be wrong. If it is not PSU, then a bad MB will be the question.

The problem is how do I isolate the issue. I can change the MB, I can change the PSU, but first I have to know what's wrong. Probably the best step is to go to a computer service. Although in my city there aren't too many that you could trust with a computer.

Yes indeed. I think the temporary solution is simple. It's time for this unit to go to a computer service and get cleaned, and after that checked for this issue. Unlike my laptops the desktops rarely get a cleaning treatment.