Have you tried creating a new project (Project -> Quick Start -> Templates -> Blank Setup) and try the minimal code in there? I did and it worked as it was meant to do.
Have you tried debugging your project and adding a watch on MY_VARIABLE just to see if it's not getting clobbered somewhere further upstream? You might want to add a MessageBox before the "If" just to confirm what's in it if the add watch route is a bit too involved or fiddly for a simple confirmation.
What version are you using? I only have 12 studio admin installed.
Peter. Smartly dressed, he still resembles an unmade bed.
InstallAware MVP
The problem was coming from language. If you do some modifications inside your setup don't forget to export then reimport all languages.
My default language was english and I was running my setup in an other language.
If you use: If variable MYVARIABLE equals TRUE
TRUE is exported for translation, so if you change the test in your setup you must export then reimport all languages before testing it or test it in default language.