I tried to update av software on an old xp 6-gig vdi (which I use to drive a 10-yo usb photo scanner on a linux virtualbox host), but avast and avg both want crazy amount of disk space, well > 1gig, when there's only 1.6g free on the vdi (avast actually refused to install, even though I had an older version on it). So after a couple of false starts I found panda antivirus, which despite the dodgy-sounding name appears to be a legit product.
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"Spread the word or you basically hate sick children."
bastard by name