Hi,
Please try to clear the Show Friendly HTTP Error Messages check box (Tool -> Internet Options -> Advanced tab -> Browsing section -> clear the Show friendly HTTP error messages check box -> OK ) and to see exact error message.
What type is your ASP.NET website, Http-Based or FileSystem-Based? If it is FileSystem-Based (using ASP.NET Development server), to better understand your issue, please confirm the following information:
If we create another type project, does it have same issue? We need to narrow down the cause of this issue.
Did you install any antivirus software which could prevent Asp.Net Development Server? If so, you can try to reconfigure your antivirus software. You can refer to http://forums.asp.net/t/1235447.aspx
I look forward to receiving your test results.