Trying to uninstall a software using the uninstall string (and running that with Exec in InitializeSetup) before installation, it doesn't wait for the un-installation to complete but proceeds to the next step of installation in Inno Setup.
I am using the following code and the software I am trying to un-install in an Installshield product:
Exec(
ExpandConstant('{pf}\InstallShield Installation Information\{{XXX8X88X-XX8X-88X8-X8XX-88X888X88888}\setup.exe'),
'-s -runfromtemp -l0x0004 -removeonly -remove', '',
SW_SHOW, ewWaitUntilTerminated, ReturnCode)