How to check for pre-requisite before bootstrappers are run within wix

I have defined a SQL server bootstrapper in the .wixproj file:

<BootstrapperFileV7 Include="Microsoft.Sql.Server.Express.10.0">
  <ProductName开发者_开发问答>Sql Server Express</ProductName>
</BootstrapperFileV7>

And the pre-requisite check in the .wxs file:

<PropertyRef Id="NETFRAMEWORK35_SP_LEVEL" />
<Condition Message="This setup requires the .NET Framework 3.5 Service Pack 1 to be installed.">
  <![CDATA[Installed OR NETFRAMEWORK35_SP_LEVEL]]>

The SQL server bootstrapper is called before the .NET check and fails as it requires .Net.

What I would like to happen is inform the user they need to to download and install .Net as it is too large to include in the installer. Is there a way to ensure that the pre-requisite is called before the bootstrapper is run?

I had a similar problem. What I did was create a native preinstaller / bootstrapper. I had it check for prerequisites and install .Net if needed then execute the wix installer file. I wrote a blog about the process at http://blog.foldertrack.com/?p=45