After WP3.1 Upgrade Site Won't Display

I use my test site at home to, well, test upgrades before moving them to https://www.fromtheranks.com. My system at home is still XP Pro SP3, running IIS 5.1, MySQL 5.1, PHP 5.2, etc.

I was/am on WP2.7.1, with Atahualpa 3.3.3 as my theme. I know, I should have upgraded before now but I’ve been very busy. :o(

I went through the upgrade steps (in brief: backing everything up, deactivating all the plugins, deleting wp-includes and replacing it from the download version, deleting wp-admin and replacing it from the download version, selectively updating wp-content files from the download version, running the “Update WordPress Database” step — which ran properly or seemed to, and then was taken to the new version of the admin page. So far, so good.

When I then tried to bring up the site, under the default WP theme, nothing. It timed out I guess. Firefox gave the error: “The page isn’t redirecting properly … it will never complete”.

So I started debugging, in Admin:
1. all the posts and pages showed up.
2. thinking maybe the old WP theme was the issue (I had activated it before the upgrade to take Atahualpa out of the loop), I switched over to the TwentyTen theme. That didn’t fix the problem.
3. I activated Akismet 2.5.3 thinking maybe WP wanted to see at least one plugin. That didn’t fix it.
4. I moved the Recent Posts widget into the Primary Widget Area thinking maybe WP needed at least one widget. That didn’t fix it.
5. I went into General Settings and resaved it, and also checked the Permalink settings and updated it to the form I had been using. Those didn’t fix the problem.
6. I checked my wp-config.php, and updated it to the new Security Keys section. Though it needed to be done, that didn’t resolve the problem.
7. Checked my index.php to make sure it didn’t get stepped on. It wasn’t.
8. I’ve gone through and checked everything I can think of, and nothing so far jumps out at me.

I can log into the Admin page easily. If I create a test post and save and then preview it, I see the TwentyTen home page. But I can’t directly call my localhosted test site as I could immediately before the upgrade.

Any thoughts, hints or “you dummy” suggestions? Unfortunately as my upgrade testing is localhosted I can’t provide a link. ???