Run uninstaller before installation

Hi

In my installation in .oninit function I use to first check whether application is already installed or not. If it is installed then a messagebox is displayed that the so and so app is already installed do you want to reinstall it. The message box has yes and no button. Now what I want is if user hits "yes" then the uninstaller first run uninstaller so as to uninstall previouly installed files and then proceed with normal installation.

Any help

Thanks

Maybe makensis.nsi in Example-folder helps you. There is function PageLeaveReinstall on it. You can use some of this code.
For example when user click "Yes" (on messagebox IDYES "label")

label:
HideWindow ;hide installer window

ClearErrors
ExecWait '...' ;run uninstaller
IfErrors ...

Delete '...' ;delete uninstaller ad other action

Quit
BringToFront