Thank you for the PM. My long standing policy is to never answer questions asked by users via PM. In your PM, you wanted to know specifically how you can uninstall IE8 and go back to IE7.
HERE is how you do it. It comes with a caveat. If you installed IE8 before you installed XP SP3, it cannot be removed unless you first uninstall XP SP3.
Alternatively there is another way to permanently disable InPrivate browsing feature by doing some registry edit. I hope you are familiar with working with Windows Registry. If you are, go HERE and follow the instructions given by lurkswithin in Response Number 6.
Either way, Make sure you first do a System Restore backup before you do anything.
Let us know how you make out. Good luck