Stop Running this script?
A script on this page is causing Inetenet Exploerer to run slowly.
If it continues to run, your computer may become unresponsive

Stop Running this script?
A script on this page is causing Inetenet Exploerer to run slowly.
if it continues to run, your computer may become unresponsive

The best explanation is from Microsoft themselves:

Error message: "A script on this page is causing Internet Explorer to run slowly" (http://support.microsoft.com/kb/175500)