XP Passwords

i have a little problem with my win XP, since i’am really a newbie in XP.okay, here is my problem: my PC is connected to a network, and when i want to access a network media, XP offered to remember the passwords.
the real problem is, some PC in network changed their password and username , and when i tried to access that PC, XP ask the password again but username box is disable, and all i can do is fill the password.
my questions is, how to delete this ‘stored’ username and passwords?