WOW! nearly did the same mistake!
I AM USING WIN XP sp3 !
My case is, that i once downloaded and installed faulty image of arma2oa with virus from web - never run it thou cause i was worried of infection! Then i got version from Impulse and when i tried to install it i got error message - "cant create file: C:\...\arma2Patch_1_07.exe"
Note i have original Arma2 installed and still playing and now got Arma2 Op Arrowhead!
Got even second version (DISC) because i was suspecting error in downloaded version, BUT Sadly never got any success!!!
Tried sfc /scannow, removing all traces of arrowhead in regedit no succes!
:868:
HEELLLPPP PLEASE!!!!!!!!!!!!!!!!!!!!!!