As i said yesterday it was kind of messy.
I uninstalled both games and then after installing arma2 and patch it to 1.07 this morning i installed operation arrowhead.
To be able to patch the game i had to add to the windows registry the path to my installation folder for arma2 (i use win7 64 bits) and maybe it was the problem yesterday cause i kept getting the "there is no arma2 installed in your computer" error.
In case someone needs it, it was here: (u have to launch regedit.exe and then find this)
HKEY_LOCAL_MACHINE \ SOFTWARE \ Wow6432Node \ Bohemia Interactive Studio \ ArmA 2
I added a new string:
MAIN ---> and in value i added my ArmA2 installation folder
After having ArmA2 installed and patched again, i launched the Operation Arrowhead installation and it auto detected the ArmA2 folder and i installed it there.
I cannot try the game yet, cause till tomorrow my copy won't be activated.
Note: The Op. Arrowhead installation applied again patch 1.07 to my arma2 game even when it was already in 1.07 version.