I've tested on a physical, non-VM machine, and it seems that the combination of XP with either IE7 and IE8 work without issue. I'll keep an eye out for any other configurations of Windows+IE that become available, but for the moment, I'm not able to debug this problem. If this turns out to be a more widespread issue, I'll try to collect more data from you and others to see if I can work out some correlation. Until then, please let us know if you see anything else that might help us start to replicate the issue here.
I've taken a closer look at this issue, and I've committed a potential fix. Please continue any discussion in http://www.sencha.com/forum/showthread.php?t=205452. Unfortunately, because I'm not able to test here, I can't be confident that this resolves the issue completely.