Modify your CSS per this thread and let's see if that fixes the issue:
http://extjs.com/forum/showthread.php?p=80794#post80794

To fix the IE Grid run-away issue
PHP Code:
Changed EXTJS Line #337 from:
.x-grid3-header-offset{padding-left:1px;width:10000px;}

to
.x-grid3-header-offset{padding-left:1px;width:auto;}