如果检测到有老版本存在就先卸载老版本才形始新的安装过程
如果检测到有老版本存在就先卸载老版本才形始新的安装过程，写了如下代码
QUOTE
Function .onInit
ReadRegStr $0 HKLM "${PRODUCT_UNINST_KEY}" "InstallDir"
${If} $0 == ""
; MessageBox MB_ICONINFORMATION|MB_OK "立即开始全新安装 $(^Name)！"
${else}
MessageBox MB_ICONQUESTION|MB_YESNO|MB_DEFBUTTON2 "检测到系统中已安装 $(^Name)，继续安装前要卸载现有版本吗？" IDNO +2
ExecWait "$0\Uninstall.exe"
${EndIf}
FunctionEnd
ReadRegStr $0 HKLM "${PRODUCT_UNINST_KEY}" "InstallDir"
${If} $0 == ""
; MessageBox MB_ICONINFORMATION|MB_OK "立即开始全新安装 $(^Name)！"
${else}
MessageBox MB_ICONQUESTION|MB_YESNO|MB_DEFBUTTON2 "检测到系统中已安装 $(^Name)，继续安装前要卸载现有版本吗？" IDNO +2
ExecWait "$0\Uninstall.exe"
${EndIf}
FunctionEnd
