Systemscan NIS2009

First I always made weekly on Sunday a complete systemscan (planned/during inactivity). I have C:, E: and D: Total 1.350.000 files. There were problems working on the computer during scan (crash and slows down) and the inactive time was over several weeks to short to complete the scan and often no threats were found. So I now make a complete systemscan monthly. But it takes  five to six hours to complete a scan, and I didn’t made a scan of my H: 600.000 files en J: 600.000 file. If I do so, it takes over ten hours to do a complete systemscan. What I’m a doing wrong? We are living in 2009 and I assume other have more files on there computer than me. Must I and they scan a whole day?  What can I do? Is Symantec working on this matter. To do a complete systemscan takes much too long!