How to get iPhone app that's no longer in the store?

If you were running Windows 7 before the reinstall, look for the following folders:

\username\My Music\iTunes\Mobile Applications
\username\My Music\iTunes\iTunes Music\Mobile Applications

If you were running Windows Vista before the reinstall, you should look for:

\username\Music\iTunes\Mobile Applications
\username\Music\iTunes\iTunes Music\Mobile Applications

If you were running Windows XP before reinstalling, look for:

\Documents and Settings\username\My Documents\My Music\iTunes\Mobile Applications
\Documents and Settings\username\My Documents\My Music\iTunes\iTunes Music\Mobile Applications

These are the default locations for your apps. If you had changed the location of your iTunes folder in your iTunes preferences, you could search your backup copy for a folder called "Mobile Applications" or for the app's name.