I just last night started having essentially the same problem with IE8, running on XP Pro SP3. It's not responding when I try to close tabs.
On the internet, it says that this problem can be caused by Add-ons but I have not had any new Add-ons recently.
I haven't either... though various security updates and patches, in my experience, do interact. The most recent is an Adobe Shockwave Flash Object dated 11/13/10 and I have that disabled.
There is a "Start Without Add-ons" mode for Internet Explorer that you can access by right clicking the IE desktop icon... not a shortcut, but the hard-wired icon on your Desktop... or you should be able to find the startup option via Start -> All Programs -> Accessories -> System Tools.
When I run without any add-ons, the browser works fine. When I've followed the step-by-step disable instructions and disabled the obvious suspects (but not everything), so far the browser still doesn't close tabs. Disabling my anti-virus software, another obvious suspect, doesn't change anything. I may need to disable everything else one by one, which will make for a long night.
I'm almost at my wits end. Came here to check out a post of mine from October [webmasterworld.com
...] and see if that problem or suggested cures might be related.
After Thanksgiving, I may look into Process Explorer as Fotiman suggests. My difficulty is compounded by an intermittent problem my ISP has been having with its DSL service, which I thought might have caused a glitch in downloading an update file or some such.
FYI, two articles I've come upon re No Add-ons mode for IE... Troubleshooting and Internet Explorerís (No Add-ons) Mode http://blogs.msdn.com/b/ie/archive/2006/07/25/678113.aspx
[blogs.msdn.com] Using the Internet Explorer 8 Add-on Manager http://windows.microsoft.com/en-US/windows-vista/Using-the-Internet-Explorer-8-Add-on-Manager
Note that the above articles may contain a combination of info for IE7, IE8, and Windows 7, but the basics have led me to the controls in IE8 on XP.