Hello there, and welcome to LinuxQuestions.org!
There aren't that many Linux viruses around, and those that do exist usually target a specific distribution, so it's harder to get a virus in that sense. Although since Ubuntu is common, I do think it is a slightly larger target.
Anywho, (correct me if I'm wrong folks) I believe ClamAV only scans for viruses that can affect Windows. The real security threats to Linux are rootkits.
It sounds like you installed ClamAV from source, but I'm not 100% sure based on the description. What you want to do is find out where you put the ClamAV file. It's probably titled something like ClamAV.tar.gz or something of the sort.
Find it, and extract the files somewhere. You can extract it in a terminal after finding the location where you downloaded it:
tar -zxvf ClamAV.tar.gz
-z is unzip
-x is extract
-v is verbose
-f is file
You can find the official ClamAV download for Ubuntu here:
The INSTALL.txt or README.txt file gives you the basic and detailed instructions to install it which (for a good portion of installation-by-source programs) are:
Type in one after the first command has been fully executed.
However, as I've stated the real threats are rootkits, which can be scanned for by using these 2 programs:
rkhunter's Official Page: http://rootkit.nl/projects/rootkit_hunter.html
chkrootkit's Official Page: http://www.chkrootkit.org/