I would like to call your attention to an alpha version of a tool that Peter Rosser wrote that will automatically repair and re-register parts of Media Center to fix some of the issues we've seen so far with Update Rollup 2 for MCE 2005. You can read about the exact steps the tool does and where to get the tool in this blog post.
Please note that this tool is not "officially" supported, but it has proven useful in some scenarios already. Also, we will be making every effort to add additional steps to it as we discover workarounds for additional issues caused by Update Rollup 2. Also, as Peter states in his blog post, if the tool does not work for you, please contact us and let us know. I've posted a list of log files that would be useful to us from Update Rollup 2. Please send them to us with a description of the problem you're seeing. My email is email@example.com, and you can contact Peter here.