From: Bailey G on
ERROR:

Could not load file or assembly
'Microsoft.SharePoint.ApplicationPages, Version=12.0.0.0,
Culture=neutral, PublicKeyToken=71e9bce111e9429c' or one of its
dependencies. Access is denied.


I'm leaving this here in case someone else experiences this.....

I had activated a feature at the root of a new portal and after a
little while I couldn't access All Site Content or Site Settings, or
even use Sign In As A Different User.

The change that I made hasn't taken effect because the app pool hadn't
recycled itself. The easiest fix for this is to go to the affected
nodes, open the web.config for that portal, add a space to the end and
then save the changes. Voila, problem solved.