Hello,
Yes, it is possible to change this behavior.
By default, WebMatrix will use the latest version of Visual Studio installed - so if you had installed Visual Studio 2008 after 2010, WebMatrix will default to running with 2008 because it considers it as the "current" version installed.
To change this, you will have to modify registry keys that Visual Studio uses to tell Windows which version is the "current" version.
These keys should be:
HKEY_CLASSES_ROOT\VisualStudio.DTE\CurVer = VisualStudio.DTE.10.0
HKEY_CLASSES_ROOT\VisualStudio.DTE\CLSID = {656D8328-93F5-41a7-A48C-B42858161F25}
The CLSID above came from HKEY_CLASSES_ROOT\VisualStudio.DTE.10.0\CLSID
Keep in mind to back up your registry before making any changes to it.
Let us know if this works for you!