what do you mean by "log on to any account that he has?" You mean access any folders or files he has shared across the network?
Not to be a smartass but the simple solution is to format your computer and put Windows XP on it
All networking from then on is a joke. I have NEVER had a problem with networking since I jumped to XP. In fact, most of the networks are damn-near plug 'n play once you get XP on all the systems.