Windows 7: W7 cannot access XP's shared drives

XP SP3 is used. The Simple File Sharing is not enable.
Ping from Windows 7 to XP goes through.
The same log-on user name and password are used for XP and Windows 7.
XP machine is visible from Windows 7.
Error message is "Unknown user name or bad password. The same message when attempting to access from another Windows 7.
Initially it was working but after an hour or so, it quits. I rebooted the router, XP machine and Windows 7 machine but the situation remains unchanged. There is no problem networking between the two W7s.
Any suggestion is appreciated.

It was working, but now it's not? What has changed on the XP or 7 computer? What security software are you using? What directories are you sharing?

Using the set up described in the link below seems to be the most reliable way to get sharing to work between Windows 7 and XP. Most any A/V software can all cause hell and havoc for sharing folders or files so there is always that possibility as well.

How to make Windows 7 work with older Windows versions for networking and file sharing.

There are many other ways to accomplish this, all I ever needed to do was to run the network set up wizard on the XP machine then set up sharing on a per folder basis then use the advanced sharing settings on the Windows 7 machine.

There is not any change I am aware of. The security software I am using is MS Security Essentials. It was working from both of the two W7s. I felt puzzled.
Edit: I added another Windows 7 to the network and tried but it does not work. If anything happened since, it appears to be at XP.

The reference appears to be how to let older machines access Windows 7. Note that there is no problem networking among W7s, and the problem is accessing older machine from Windows 7. It appears to me that something with XP needs be changed. Is this valid logic? By the way there is no problem accessing W95 and W98SE from Windows 7 on the same network.