I discovered a few easy steps that can allow you to do this. Note: I had already converted (most) of the project files to use .NET 2.0. If you are using Ajax extensions you will have to make your web.config align with this as well. Do this first in VS2008. Obviously if you are using any .NET 3.0 or .NET 3.5 features, this isn't going to work for you...
Next open the csproj file you want to change in Notepad.
Near the top is the Project node. If this contains ToolsVersion="3.5" remove this attribute:
<Project ToolsVersion="3.5" DefaultTargets=...
becomes
<Project DefaultTargets=...
A few lines down there is a <ProductVersion> node, change the value:
<ProductVersion> 9.0.21022 </ ProductVersion>
becomes
<ProductVersion >8.0.50727 </ ProductVersion>
Near the bottom of the file, look for the
<Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
becomes
<Import Project="$(MSBuildBinPath)\Microsoft.CSharp.targets" />
If there is another <Import node with an MSBuildExtensionsPath, delete this entire line.
Save the file and you should then be able to to open the file in VS2005. Don't double click on the file if you have both versions of Visual Studio installed, you will likely get the default VS2008 to load it.
