Web Shield scans don't incorporate the DeepScreen (which would put a heavy load on the system), also the file the web shield is alerting on within the installation file is 7z.exe.
Original alert:
02/12/2013 12:12:35 hXXp://dl.7-zip.org/7z932.exe|>$INSTDIR\7z.exe [L] Win32:Evo-gen [Susp] (0)
During subsequent scanning in the partial download file in the temp folder it found.
02/12/2013 12:20:14 C:\Documents and Settings\UserName\Local Settings\Temp\aw0b0noL.exe.part|>$INSTDIR\7z.exe [L] Win32:Evo-gen [Susp] (0)
02/12/2013 12:20:46 C:\Documents and Settings\UserName\Local Settings\Temp\aw0b0noL.exe.part|>$INSTDIR\7z.sfx [L] Win32:Evo-gen [Susp] (0)