Hi Paul,
Malwarebytes has a completely free version.
It does not run in the back ground. ie: Real time.
There's no probs with installing it & running it when you require.
Just keep the defs updated.
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https://www.malwarebytes.org/
&...Even though AVAST free is a little bit of a PITB, it's still a good anti-virus app.
The free version does real time scanning & auto updates.
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https://www.avast.com/en-au/index
The advert/s pop ups can be a tad annoying, but that's what "they" included in it.
They cannot be turned off, but you can in the paid for version.
Not worth the $$$ IMO.
When installing, be sure to UN-check the Browser Cleanup, Grim Fighter & the so called "App Updater" features.
They are not needed in an anti-virus program.
SUPERAntispyware is also recommended.
Similar features to MWB, but for spyware.
Does not run in the back ground & you perform the (manual) updates.
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http://www.superantispyware.com/
All these run perfectly with no conflicts.