Vista - Help Needed: Vista cannot connect to XP on network

I've 2 notebooks, Vista and XP.

Both can connect to the internet wireless via a router with no problem. The workgroup names, user accounts are also identical for both. I can also see both notebooks on the network. Firewall shouldn't be a problem as I've allowed access for the IP address and both notebooks are already visible on the network.

(A) When I try to access the Vista from XP, it prompted a user name and password. I entered accordingly and is able to access the Vista notebook. No problem.

(B) However vice versa, when I try to access the XP from Vista, the user name and password I entered doesn't work!!! I get the error message: Logon unsuccessful: Windows is unable to log you on. Be sure that your user name and password are correct.

I'm sure I entered the info correctly. Is there a specified format to enter the password in Vista?

Any help will be appreciated. Thks!