MSBuild Error when building VS2008 Projects

I've been playing around with MSBuild and CruiseControl.NET and ran into a problem trying to build projects built using VS2008.

File format version is not recognized.  MSBuild can only read solution files between versions 7.0 and 9.0, inclusive.

Turned out I was targeting the v2.0.50727 version of MSBuild in the ccnet.config file

<tasks>
  <msbuild>
    <executable>C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\MSBuild.exe</executable>

Rather than the 3.5 version

<tasks>
  <msbuild>
    <executable>C:\Windows\Microsoft.NET\Framework\v3.5\MSBuild.exe</executable>