Slow Manual Scans

I have Norton Antivirus 11.1.2 (17) as part of the Norton Internet Security Suite for MAC and have the latest Snow Leopard version on my computer.  About a month ago I realized my antivirus program no longer seemed to be caching scanned files. When I ran a manual scan of my computer more than once between updates, the scan went as slow as the first time it ran because it was scanning every file again.

 

Before this started, any additional manual scans took only a few minutes after I ran a longer complete scan of all the files. Once I updated my Norton Antivirus program, it  knew to do a complete scan on the first scan afterwards.

 

I notice a cache file at Library/caches/com.symantec.nortonantivirus.  But I also noticed the "date modified" column associated with that file is Mar 3, 2011 at 5:22 PM.  That happens to be the exact time I renewed my subscription.  So i thought it might not be a good idea to delete that file since it might be telling Norton I'm paid up for 12 months from that date.  

 

Any suggestions as to why my subsequent manual scans between updates no longer skip previously scanned files will be appreciated.  

 

Thanks.