No passwords needed for that. I've been sharing with my XP machine for over 6 months and I do not need to use any passwords.
You only need to set up basic file sharing on the XP machine and then enable the appropiate sharing tabs in Windows 7. You only need to worrie about passwords if you set them up before hand.
It's so easy I'm not about to go into details on it because it will only complicate something that is already beyond simple.
Shibu, this is easier that you are making it out to be and there are no tricks to it. If you can't access the XP machine then work with the sharing settings in XP until you can do so. It's that simple.