You have a multilingual application, and you want the URL of a page to be different for different languages, to account for SEO. The URL for the contact page for example should look like http://something.com/en/contact in english, and http://something.com/de/contact in german. This tutorial describes how to make it happen.
Note that currently selected language is always a part of the URL, and thus available to the application through $_GET. You don't have to use sessions variables to keep track of the language if you don't want to.
Installing WAMP on XP was pretty easy; W7 is not so trivial. It's not difficult once you know, but it took me best part of a day to figure out and required gleaning various bits of information from around the web; this tutorial brings it all together in one place to (hopefully) make other peoples lives that bit easier. It does assume that you are reasonably happy configuring Apache, PHP, and MySQL and mainly deals with the W7 quirks.