Network Sharing

It's likely looking for the admin / primary user account password. (for the target computer, not the one you're trying to connect from.)

If you are running windows 7, you may need to modify the sharing and security permissions. I've had this problem recently, two Win7 computers and two WinXP computers. You will want to make sure 'Users' and 'Everyone' are added and have permissions in both the security and advanced sharing permissions. If not, just add them. Let me know if you need instructions.

Edit:Posting instructions anyway.
Right click the shared folder, click 'Properties', then click the 'Security' tab. Check to see if the 'Users' or 'Everyone' groups are listed. If not, click the 'Edit' button. On the next window, click add, and another window will come up asking you to type in a group name. Type in whichever one of the two above were not listed, then click OK. That window will close automatically, and now you should see the newly added groups listed. You can modify their permission control, but I would recommend leaving it to 'read & execute', 'list folder contents' and 'read' so no one can actively change anything in the shared folder. This is up to you though. Once this is done, click OK, and that window will close, leaving the Properties window. Click on the 'Sharing' tab, then 'Advanced Sharing'. A window will come up, you will want to click the 'Permissions' button. The process will be the same here as before when it comes to adding a user group policy. You should be green now as far as sharing. If you still hit a road block, try removing yourself from the 'Home Group' in the network and sharing center. I had to do this on my Win7 computers for the WinXP computers to have access.

This worked for me so I thought I would post it for you and anyone else who happens to read this post and has the same issue. If it doesn't work, let me know.