strange problem with manual scan on norton 2010

hi,

 

recent I upgrad to norton 2010 on my winxp pc. I usually turn off the auto protect (because it slow down my pc) and do manual scan (right click and select scan now) on a file to know whether any problem with it.

 

I found the problem yesterday when I try to scan a zip file . there're 4 files in it . I right click and select scan now . after few minute norton report no problem (showing 4 files scan).

 

latter when my schedule scan run , norton report problem with the zip file and found virus in it .

 

at that time I suspect problem with the right click manual scan . so I get the same zip file again to scan it . and again  norton report no  problem . this time I manual click the schedule scan now and again noprton report virus found .

 

I verify the zip file again with virustotal.com and it return me report that the file inded contain virus.

 

what going on here ? is this somekind a bug?