I had the same problem myself on a fresh install.
Checking the clamd log at:
"C:\Program Files (x86)\SmarterTools\SmarterMail\Service\Clam\Log\clamd.log"
showed errors each time it tried to start
Sat Apr 18 11:00:38 2015 -> +++ Started at Sat Apr 18 11:00:38 2015
Sat Apr 18 11:00:38 2015 -> clamd daemon 0.97.6 (OS: win32, ARCH: i386, CPU: i386)
Sat Apr 18 11:00:38 2015 -> Log file size limited to 1048576 bytes.
Sat Apr 18 11:00:38 2015 -> Reading databases from C:\PROGRA~2\SMARTE~1\SMARTE~1\Service\Clam\share\clamav
Sat Apr 18 11:00:38 2015 -> Not loading PUA signatures.
Sat Apr 18 11:00:38 2015 -> Bytecode: Security mode set to "TrustSigned".
Sat Apr 18 11:00:38 2015 -> ERROR: Can't open file or directory
Running clamd manually in debug mode with the following command:
C:\"Program Files (x86)"\SmarterTools\SmarterMail\Service\Clam\bin\clamd --debug -c "C:\Program Files (x86)\SmarterTools\SmarterMail\Service\Clam\etc\clamd.conf"
Gave an error saying there were no database files, and sure enough the configured directory Clam\Share\ClamAV was empty.
I downloaded the database files from from http //database.clamav.net/main.cvd and daily.cvd to the ClamAV directory, and clamd started okay after that.
I don't know why the SmarterMail install didn't install the database files