Force.com IDE - Password Required

Symptom

On startup, Eclipse Force.com IDE sits at "Pre-fetch SObjects" step for a long time, and then asks for username/password.

Possible Causes
Java seems to cause this problem.

First time I experienced it, it appeared that Java had become corrupted.
Second time was immediately after installing a Java update.

Update: this happened a third time and messing around with Java didn't fix it.

It turns out that the  Eclipse proxy settings screen is a bit buggy. A lot of corporate firewalls don't support SOCKS so this has to be disabled. When selecting Manual settings, it appears to be impossible to disable the SOCKS proxy settings. However selecting the setting and clicking Clear seems to fix the problem.

http://stackoverflow.com/questions/5857499/how-do-i-have-to-configure-the-proxy-settings-so-eclipse-can-download-new-plugin

Possible Resolutions

1. Check and correct Java proxy settings. It appears these can be lost sometimes

Start | Settings | Control Panel | Java | General tab | Network Settings...
Tick Use proxy server and enter the address and port
Click OK until all dialogs closed

If not sure what to use, see your Eclipse proxy settings (Window | Preferences | General | Network Connections)

2. Repair Java
3. If this doesn't work, uninstall and reinstall
4. It may be necessary to go back to a previous version