I suggest taking a look at the ClamAV mail-list.
I found this:
http://www.mail-archive.com/clamav-users@lists.sourceforge.net/msg01636.html-------- from the above thread ------------
> I had to change my mirror for ClamAV this morning as well. It
> checked for updates at 6:18am CST with no error, when it checked
> again at 8:18am CST it was reporting a bad checksum for viruses.db2
>
> I changed my mirror and all is well again. I guess they have a
> corrupt file up on the mirror clamav.elektrapro.com
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