Domain based network...keeps asking for password...

Hi,
Here's the problem...I'm on a LAN, which works on a domain, not a workgroup...all 4 PC's using XP professional.
The problem came up when we added the 4th PC to the LAN...
Now we have a problem sharing files. Whenever I try to access any of the other 3 PC's, it asks me for username and password...what user and pass?!?! I didn't set any password...I actually do want to password protect some of the folders...but I don't know how!! In win98 you could simply set a pass when you shared a folder...but here in XP i have to deal with Permissions...and when I go in the Advanced part of the Permissions Dialogue, I only see local users and groups...in the Locations Tab too, I only see my own PC.
Where am I going wrong?
I am sitting here, refreshing my browser for anyone to give me any tips!!!
Please help! thanks!

are you sure that you added this PC correctly to the domain? did you check the domain controller to see if this PC is indeed added to the directory?

If you can only see local users/groups then the pc has not joined the domain. If you are part of a domain you will be able to see the directory.

just to agree, if all you can see is your local users then you are likely not on the domain, if you show to be on the domain, goto the domain controller, delete this computers name from the 'Computers' folder, then on the PC leave the domain and join it again, make sure you reboot afterwards as always. Then when you login in make sure you are logging into the domain and not the computer. On the login prompt select "Options" and in "logon to" make sure the domain is selected not (local computer) That should be all there is to it.