The next time this happens, use the Task Manager to 'log off' (Ctrl-Shft-Esc > Users > Your Account > Logoff). Now log on. Is the problem resolved?
If this resolves the problem, I recommend downloading and installing TweakUI from Microsoft:
http://www.microsoft.com/windowsxp/downloads/powertoys/xppowertoys.mspx
Open TweakUI:
Click About > Policy > Run Group Policy Editor > Computer Configuration > Administrative Templates > System > Logon
Right click 'Always use classic logon', click 'Properties', click 'enabled', click 'OK', and exit Group Policy Editor.
The next time you boot up the computer, you will see a logon dialogue box. If not password protected, you can simply hit 'Enter'.
I don't know why this convoluted procedure works, but it does. Please reply by subsequent post if your problem remains unresolved.
Rick/Portland, OR