I just tested out ESET NOD32 antivirus on a folder full of old DOS viruses, basically to see if it would still detect very old signatures. Turns out, it doesn't and ESET is not alone. Most of the big-name programs don't include the old stuff in their definitions databases.
I not only rely on my modern virus scanner to protect from new threats, but old ones as well since I run a file server with a bunch of old DOS and 16-bit Windows applications stored on them. Most are from reputable sources (i.e.: I've created the images of disks or CDs myself from original media), others purport to be original images but are downloaded from the internet, so you never really know.
What's the solution here to be absolutely safe? Run a separate NAS with an old enough operating system to run the old virus scanners? For those of you who might remember back to the 1980's and 1990's, there were some viruses could do actual physical damage to old machines or at the very least, render BIOS chips unusable. There were even those which would cause CRT monitors to over-scan or stress components to the point of failure (I don't recall the exact machine(s)/virus code right at the moment).