Thank you everyone for their suggestions. I thought I'd respond with my findings in case there is anyone else looking for a similar product.
After trialing a few different packages, I've settled with
ESET NOD32 Antivirus. It does exactly what I need it to do all while sitting in the background quietly.
As a bonus, the latest version also runs on Windows XP (which I have one machine running legacy software) and on Windows Server. Updates are rolled out several times a day and I particularly like their
update information page which lets you see exactly which viruses etc... are being detected by that pattern file version (something a lot of AV providers don't provide, you just have to trust they get the latest stuff).
It uses a relatively small amount of resources and doesn't seem to effect the general speed of things even with real-time file scanning enabled.
Pricing is also very competitive and you can choose exactly the number of licenses you need (rather than forking out for a 5-pack if you only need 3).
I like it