How to dynamicly Modify InstallDir？

in the general section, i can set the InstallDir and Caption of Install App, for example:
/////
Caption "SW"
OutFile "SW.exe"
InstallDir "$PROGRAMFILES\SW\"
////

in this case , the directory page show "$PROGRAMFILES\SW\" in the install file-selecting edit control. but i want to check if main program has been
installed in the machine, if true, i will query relative
info in regedit table to get the install dir.
how can i implement it?

See NSIS manual,