a 3rd factor might be that your antivirus program has its
'resident shield' program on.
this means that the antivirus program might have been programmed
to scan the whole computer once it runs, or that it has been
programmed to scan a file when its being opened, or when a program
is executed.
try disabling these options in your antivirus program if
able.
if not, i highly recommend avg free anti-virus.
avg comes with scan on start and resident shield too, but
you can select these operations to run at your discretion, as in
they can either be on or off.