On-Access Scanner just turned off and I can't turn it on

Hi,

I'm brand new to the forum.

Have been using the free Sophos Anti-virus product for Macs for 2 years now.  This morning I got a message saying "on-access scanner is off".    I went to preferences to try and turn it on but it wouldn't allow me to do this.

Any suggestions would be appreciated.

I'm not even sure what this on-access scanner does, but I assume I would want it on.

Thanks,

daBash