F-Protect (or F-Prot) for DOS is the only antivirus I have used
since 1994. It is free for private use
ftp://ftp1.mirrors.f-prot.com/pub/f-prot.zip
Free malware signature file updates are available every few months.
That may not sound sufficiently frequent but the program has very good
heuristic scanning capabilities that allow it to identify suspicious
or dangerous files before they have been publicly identified. (I
have tested this myself.)
Caveat: I surf without Javascript or Java and I do all email by
telneting to a Unix account so I have less stringent AV needs
than usual. For most computer users the free DOS version of F-Prot
will not be adequate by itself. But as a free supplement to a more
standard AV, why not?