I haven't found the root cause yet, but here's how to get them back.
- Open the Task Manager. Find TSVNCache.exe and click End Process.
- Shutdown and then restart Windows Explorer. Go to Start > Shutdown. Hold down the Shift+Alt keys and then click the Cancel button. To bring up the Task Manager, hold down the Ctrl+Shift keys and then click Esc. Now, in the Task Manager, go to File > New Task and type in explorer.exe. Supposedly, this is the "proper" way to kill and restart the Windows Explorer process.
UPDATE: In XP Pro, all I needed to do was end TSVNCache.exe and then I could refresh Windows Explorer and the overlays came back.