IE6 inconsistent between machines

Hoping someone can help solve a mystery. I'm developing a fairly simple site for a client. To save dev time, we purchased a template from freesitetemplates.com and I modified it somewhat. The site in development is here:

http://birdhouse.org/~bob/osmo2/

The site looks fine in every browser I've tested it on (Win/IE6 and Mac/IE5, several Mozilla variants, and Safari). But when my client views the site in Win/IE6, he sees a big gap in the middle of the design. Here's a partial screenshot he provided:

http://birdhouse.org/~bob/gap.jpg

But when I look at it in Win/IE6, the gap is not there. I've had him empty his cache, and I've tried changing the URL to defeat any proxy cache from his ISP. No luck. In fact, he sees the gap on two different Win/IE6 machines, while I *don't* see it on two different Win/IE6 machines.

Frustrated, I completely discarded the purchased HTML template (keeping the graphics) and rebuilt the site from scratch in Dreamweaver, so it's now "virgin code." The problem persists.

Utterly mystifying. Two questions:

1) Does anyone else out there see the gap?

2) Has anyone seen this kind of inconsistent behavior with the same exact browser running on different machines?

Thanks for any suggestions,
Scot

Looks like I have a clue to fixing this now - Windows' accessibility options allow users to set the screen dpi from the default to 120, which breaks some layouts (Screen properties, Settings, Advanced). I can finally duplicate the problem (and hopefully fix). Just wanted to post this here for the archives, in case people hit similar issues.