Thanks to  : Abdulla AbdelHaq
Source : http://weblogs.asp.net/abdullaabdelhaq/archive/2009/06/01/VS-Debug-Problem-with-IE8.aspx
 
How VS debugger could be crashed with IE8?
If you opened multiple instances of IE8 and you attempt to debug your project, you mostly will have the issue where VS debugger just stops and ignores your break points!
Why was that?
Well, IE 8 has a feature called Loosely-Coupled Internet Explorer (LCIE) which results in IE running across multiple processes. 
http://www.microsoft.com/windows/internet-explorer/beta/readiness/developers-existing.aspx#lcie
Older versions of the Visual Studio Debugger get confused by this and cannot figure out how to attach to the correct process. 
To overcome this issue, you need to disable the process growth feature of LCIE by follow the below steps:
1)  Open RegEdit
2)  Browse to HKEY_LOCALMACHINE -> SOFTWARE -> Microsoft -> Internet Explorer -> Main
3)  Add a dword under this key called TabProcGrowth
4)  Set TabProcGrowth to 0
 If you run into the same problem on Vista or newer, you will also need to turn off protected mode.
And then go a head and start debugging your code.