When I attempt to download the Ubuntu One client for Windows at https://one.ubuntu.com/downloads/windows/install/, I get a bad .exe file. Examining the file reveals that it's actually HTML, and not an real executable, or even a binary file at all. Opening it with a browser suggests a possible OpenID problem, but the problem still occurs when I am logged in. I've attached the file.
The Ask Ubuntu user iweb (http://askubuntu.com/users/83587/iweb) discovered this bug (see http://askubuntu.com/questions/176740/how-to-fix-16-bit-ms-dos-subsystem-illegal-instruction-installing-u1-on-w); I was able to reproduce it. According to information provided by iweb, in Windows XP (and possibly other Windows OSes) attempting to execute the file produces an error message box titled "16-bit MS-DOS Subsystem" and containing the text "The NTVDM CPU has encountered an illegal instruction." So any current or future bug reports (or other mentions) of this error in connection with the Ubuntu One installer are likely the same bug as this.
For convenience, this is the complete contents of the downloaded .exe file:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
<title>OpenID Authentication Required</title>
border: 1px solid red;
border: 1px solid yellow;
<body onload="document.getElementById('continue_btn').style.visibility='hidden'; document.oid_form.submit();">
<h1>OpenID Authentication Required</h1>
<div class="message">Authorization is required to access <a href="https://chinstrap.canonical.com/~chriss/ubuntuone-3.0.2b-windows-installer-signed.exe">https://chinstrap.canonical.com/~chriss/ubuntuone-3.0.2b-windows-installer-signed.exe</a></div>
<form method="post" action="https://chinstrap.canonical.com/openid/+login" name="oid_form">
<input type="hidden" name="openid_identifier" value="login.ubuntu.com">
<input id="continue_btn" type="submit" value="Continue">