I'm surprised that the W-10 upgrade process actually noticed that your hardware is too old. That must be a recent change, as most people don't discover that their hardware is not compatible until W-10 is installed and then doesn't work properly.
Look at the following URL which is the one for downloading device drivers for your ASUS P5Q motherboard. Note the absence of drivers for W-10. P5Q | Motherboards | ASUS Global
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Your processor model is not suitable for Windows® 10. The support for Windows® 10 is available from 3rd generation processors.