Usually a random 8 letter file like vuminuzi.dll is an indication of a vundu infection. Especially when running from the Windows\system32 folder. You may consider visiting a malware removal forum, Malwarebytes might remove it but not all of it. Looks pretty new Google doesn't have any information yet.

Run an updated malwarebytes full scan and a superantispyware scan let them clean what is found then have your avast scan it again it probably asked if you want to delete or quarantine, go with quarantine when asked that. The reason for that is should you need to add this back in it is still available but it is locked away

Several competing products are rated higher than Avast, you may want to give one a try. If you do, remember to uninstall Avast before installing another program. Having two at the same time can be a problem. Here is a link that might be useful: PC Mag Anti Virus ratings

The 20 V reading between Rc and Rh doesn't make sense unless the Nest thermostat is loading down the thermostat output. Try measuring the voltage between Rc and Rh with the wires disconnected from the Nest. The 50 V reading between Rc (which is connected to the AC transformer.) and the W which has a path to the boiler transformer supports my theory that the two transformers are power by two different legs of the 120 V supply. Rh and Rc are 180 degrees out of phase so instead of reading zero volts, you are reading the difference of 24 V - (-24 V) which adds up to about 50 V. I am surprised the Nest is working properly and has survived this configuration with two transformers. Maybe the Nest engineers thought about this possibility and designed their circuit to withstand these voltages. It would be interesting to see the internal circuit. But unfortunately Nest does not publish that information. You may be getting an error because the voltage between Rh and C is zero. The Nest may have a detection circuit that reports this as an error. Nest has a power connector device which can be used when a C wire is not available. You should be able to install the power connector on the boiler side and eliminate the C wire. Then you can restore the wire back to as the fan control. I don't know if this will work but it is worth investigating. Here is the power connector installation video: How to Install a Nest Power Connector for Your Nest Thermostat