Problems accessing shared folders

I don't really understand the security principles or access rules in Windows
XP. When I try to access a shared folder or resource (printer) on one computer
from another in my same workgroup, it asks for a user name and password.
Sometimes I enter the
information and it accepts it, sometimes it does not. Sometimes, I want to
change the user I am accessing as, but I don't know how to do that. If I have
not told it to "Remember my password" restarting my computer will let me enter
another account on the remote system, but if it is set to remember my
password, I seem completely out of options (because it just logs in again as
the same user automatically and never even asks for a new user name).

Here are my questions:

1. If I am accessing another computer, but want to change which user I am
accessing it as (to gain access to other folders) how can I do that?

2. Is there some way to force it ask for another user name and password?

3. Is there a way to enter it directly into the Address bar in an Explorer
window?

4. Why, on some computers that do have sharing enabled, does it still refuse
all users from accessing it remotely, even though I know for certain that I am
entering the correct user name and password?

Thanks for any help or advice,
Colin