Disabled button with a alert is clickable in IE8 (browser compatibility Issue in Asp.net website)

I have disabled a link button and image button(which is inside a link button so that I can show a Alert) and it works fine in Chrome, but when I open it in IE8 the image is disabled but when i click it it shows the popup(‘Are you sure you want to delete this User?’). Why am I getting this error in IE8 and IE7, how can I fix this.