Hi Lizzie Bell and welcome to BleepingComputer!
First of all, I will move this topic to a more appropriate forum.
Most likely there is an AVG leftover setting in the Windows Security Center. The simplest solution is to run AVG Remover
I recommend however, not to use Combofix unsupervised. This is not a regular antimalware tool that can be run as on-demand scanner. It is pretty powerful and can cause serious problems in some cases.
"Now faith is the substance of things hoped for, the evidence of things not seen."
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Malware analyst @ Emsisoft