The change from Report Only to Move To Quarantine will be effected the next time you scan. ClamWin will move the infected file to quarantine then.
I like to verify that a file actually contains a virus before I remove/quarantine it. If it is an important file, and ClamWin falsely reports a virus (a false positive), if it quarantines the file, you will lose access to it. ClamWin now has protection for false positives on Windows system files--if you are using Windows Vista or Windows 7. If ClamWin tells you several files are infected with the same virus, that is often the sign of a false positive. False positives do not happen very often though.
You can verify a file is infected by finding it on your computer and uploading it to the Jotti or Virus Total online scanning services. Jotti is located at http://virusscan.jotti.org/en
on the web, and Virus Total is located at http://www.virustotal.com/
on the web.
If ClamWin is your only virus scanner, you should also use the Clam Sentinel program, which adds a real-time scanning capability to ClamWin. Otherwise, since ClamWin by itself does not scan in real-time (it is only an on-demand scanner), you can get a virus and not know about it until you do a manual or scheduled scan with ClamWin. Sentinel is available at http://sourceforge.net/projects/clamsentinel/
on the web. ClamWin must be installed before you install Sentinel. When installing Sentinel, select it to Run on startup. After you are familiar with it, you can select it to Monitor the system for new malware, but you will also be notified of all changes to your system, which sometimes makes it very "busy." The next version will let you stop this.