One possibility is that you are trying to open up a web application project (WAP) in VS2005 but don't have the WAP add-on. VS2005 doesn't support WAP out of the box but you can install either SP1 or a knowledge base article and then the WAP add-on. Look here for more information: http://webproject.scottgu.com/
Another possibility is that you don't have the right version of VS express installed. With the express editions there's:
- Visual Basic 2005 Express Edition
- Visual C# 2005 Express Edition
- Visual C++ 2005 Express Edition
- Visual Web Developer 2005 Express Edition
You can only open a csproj file with Visual C# or the full/professional version of Visual Studio 2005. Visual Web Developer can build web site projects in both VB
and C# but doesn't work with application projects (.csproj or .vbproj). You likely need to install Visual C# 2005 Express in order to open the project.