We stopped distributing our offline documentation in the main installation package as it is larger than the product itself and not everyone ends up using it.
The documentation you downloaded targets the VS 2010/2012 integrated help system (MS Help 3) and should not interfere with VS 2008 in any way.
I've attached a set of instructions document from one of our other product's documentation that should help you deploy our WPF documentation flawlessly. Just remember to use the "uninstall" batch file before you start (incase you initially used the batch files to deploy our offline docs).
Let me know if that helps
All the best,
the Telerik team