If you are getting "Parser Error Message: Access is denied: 'Some Assembly'." and "Source Error: < add assembly = <
* > / >" sometimes when you are running your web applications it maybe because of index server. Here are some tips to sort it... Easiest being stopping Index Server.
http://support.microsoft.com/default.aspx?scid=kb;en-us;329065
Care of http://blogs.wwwcoder.com/deepak/archive/2004/10/14/885.aspx