There are mainly 3 ways to get to your Vsnet project file to edit it. (disclaimer: I am running Vsnet2008 and haven't tried earlier nor later versions)
1) Open windows explorer and navigate to your project folder. Open the project file (.csproj/.vbproj/...) in your favourite xml editor.
2) Install a tool that gives you an "Open folder in windows explorer" context menu item in the Solution explorer. If I recall correctly the tool I am referring to is Microsoft's Team foundation server power tools. Please correct me if I am wrong.
3) Unload the project. Use the context menu in Solution explorer to edit the project file. Load the project again.