I have Vista which is some different & I have another backup method So I have system restore disabled.I understand many XP users do the same.
If you have never disabled it then it should be creating restore points automatically.If this is the case then use a restore point before the problem occured.Here's a MS article on System Restore for XP:

http://www.microsoft.com/windowsxp/u...w_03may19.mspx

If this doen't fix your OS then I will suggest more.
Doing the opposite of this may fix the constant re logon. "Disable" instead of "Automatic"
Enable the RunAs service on a Windows 2000-based computer, or the Secondary Logon service on a Windows XP-based computer
The service that is required to run the Run As command is, by default, started automatically when you install Windows. However, the service can be disabled by an administrator. If the service has been disabled, you can re-enable it by following these steps: 1. Log on to the computer as Administrator, or log on as a user with administrative permissions.
2. Right-click the My Computer icon, and then click Manage.
3. In Computer Management, expand the Services and Applications node, and then click Services.
4. Perform one of the following steps, depending on which operating system you have:• Windows 2000: In the details pane, right-click the RunAs service, and then click Properties.
• Windows XP: In the details pane, right-click the Secondary Logon service, and then click Properties.

5. In the Properties dialog box, set Startup type to Automatic, and then click Start.
6. After the computer starts the service, click OK to close the Properties dialog box, and then close Computer Management.