printer problem

hi! i have a printer sharing problem. i have a network using windows XP. one of the computers has a printer connected and shared. when a user from another computer tryes to print either it asks for user name and password or says access denied. it's the same if you try to access another share like C$. i have checked all the permissions and everything seems ok. the strange thing that it is the only one doing this. i really don't know what to look after anymore. maybe someone can give me a hint. thx all.