Help sought diagnosing hang after login

I'd like help diagnosing a problem that has occurred twice in the past month:

I boot, log in, and then the system hangs before log-in is complete. No icons appear in the system tray. The spinning circle hour glass figure stays on screen. I can move the mouse cursor, but can't open programs. Ctrl-alt-delete has no effect.

After ten or fifteen minutes, I have to do a hard reset ... and the computer boots and logs in normally afterward.

I'm running Windows 7 Pro 64. The problem occurred the first time before I installed SP1, and again this morning with SP1 installed.

I've checked event viewer to search for the root of the problem, and can't find anything amiss in the logs ... but that might be because I don't know what to look for in the logs, too.

Can anyone offer me advice on how to troubleshoot this problem?

Issues with Service Packs mostly occur because of malware or with older drivers. Please scan for malware and update your drivers in Device Manager.

Beyond that, please provide this info (even though you're not reporting BSOD's): http://www.sevenforums.com/crashes-debugging/96879-blue-screen-death-bsod-posting-instructions.html