Check the values in /magentofolder/app/etc/local.xml and ensure all is good.
Log in and check the values at System > configuration > Web > Unsecure URL (and Secure URL) - make sure they are good (they should be, otherwise you’d not be able to log in very easily.
Go to System > configuration > Advanced > Admin and set the value of ‘use custom admin url’ to 'No' and remove any values, if any, in the URL box underneath it. Then save config.
If all these settings are correct, and you’re seeing admin/admin, then check:
System > Configuration > General > Web > URL options > Add store code to URL
There you must have set its value 'Yes'.
What this does is on the front end it includes the store code (actually store view code) to the URL for mutli-store setups, so that you can access a store like this : http://xyz.com/index.php/storecode1/ or http://xyz.com/index.php/storecode2/ ... etc.
But unfortunately it also does it for the admin (which has store code admin) - therefore, the path to your admin is now admin/admin - the first being the store code, the second being the path to serve the application. And so, some extensions that don’t use dynamic admin URL path won’t work - probably you’ll get a 404 where you expect your content to be in the main body of the page.
Enabling the second option in that admin (System > configuration > General > Web > URL options), i.e. "Redirect to Base URL if requested URL doesn’t match it" should fix all the problem.
If it still does not fix your problem then the last option to fix it to turn 'No' for 'Add Store Code to URLs' option.