Recently i bought myself a new computer and needed to reinstall the Microsoft Sql Server 2008 on it. Time and time again i ran into the error that: 

” is not a valid login or you do not have permission

This bothered me insanely, i tried to reboot the computer, started the installation numerous times and never found the problem. I changed account, i copied the files to hard drive - nothing solved this. I finaly ran across a small post on the interweb saying that someone might think that it was caused by the fact that the computer name was the same as my windows account name. I rapidly opened my control panel, and voila, my computer name was exactly the same as my windows user name. I changed the computername and tried to install again - no errors!

So... before you start trying all the things i did, please verify that your computers name is different from the windows user name.