Read from a text file during installation?

How would you do this? I want to be able to read from a text file in the install directory - it contains the value representing the current version. If the version is greater than what you are installing I want to then stop the installation.

I am stuck! Where do I start?

There are several ways to do that, all described in nsis documentation, e.g. FileOpen/FileRead, ReadINIStr, ConfigRead.