knowing that Vista is crap

Isn't IE8 still in beta?

IE8 sucks

Not that I entirely agree with you there, JAISP but from today you can download the release candidate for Windows 7 from Microsoft and run it on a "free" license until June 2010. Worth a look, eh?

IE8 went on general release 19th March, 2009. It wasn't included in all Windows Updates, but rolled out in Vista updates.

Hmm.... weeuuurlll, that aside (!!), are you using an htaccess redirect for your www.domain/domain issue? It might resolve the problem.

But I wants to give mr. Gates a little credit: If you have upgraded from IE7 to IE8, then its possible to get back to IE7 by uninstalling IE8 through control panel on windows Pc's

Well I finally got around to installing IE8 on a xp3 machine.
I don't see a issue on at least three sites.
Including registering on your site. Possibly it is the referrer check that really is a issue.
As well as what internet security software is used???

Microsoft released Internet Explorer 8 (IE8) in March, and although its deployment has been slow, the new browser has created a potential concern for designers and developers.

The new browser introduces a slew of fundamental improvements, including security, ease of use, and support for RSS, CSS, and Ajax. Most noteworthy in IE8, however, are the significant changes in the way pages are rendered. In contrast to previous versions of IE, Microsoft has made a gallant effort in IE8 to render pages in greater conformity to web standards.

ie8-compatview

Although standardization enables consistent development across browser platforms, sites designed for older versions of IE may not display as intended in IE8. (Learn about rendering in IE at Microsoft’s IE Compatibility Center.)

To mitigate display issues, Microsoft has introduced in IE8 a “compatibility view” that enables sites to revert back to IE7 rendering. It’s a quick fix that enables webmasters to punt on the IE8 rendering changes. To force display in compatibility view, simply add the following meta-element to your pages:

<meta http-equiv=”X-UA-Compatible” content=”IE=EmulateIE7"/>

Since other browsers will ignore this code, Microsoft suggests this approach has the best blend of allowing web developers to easily write code to interoperable web standards while not causing compatibility problems with current content.

You can also address this issue by adding HTTP Response Headers in IIS6 or IIS7 [read]. This is very convenient for any web developers that have static sites.