Dual boot, XP MCE, and Vista problem kinda

I had to reformat and reinstall XP MCE, I created a partition and installed Vista Beta 2. After playing with Vista and Media Player 11 I decided I wanted to download and use Media player 11 on MCE, I went to install it on the XP MCE partition and it told me that the version of windows required was MCE 2005, I checked it out and the reinstall was only for XP Pro, now I check the disc and MCE is there, but I can not reinstall it with out losing Vista How can I upgrade the XP Pro back to MCE from the disk without destroying access to Vista, Vista installs a Boot folder to the boot partition to give you the option of booting to Vista or a previous version of Windows. M$ says to reinstall XP MCE but there are a lot of horror stories around already about losing vista if you do. When I put the MCE disk in I am given the option to upgrade, which I am tempted to do as this is supposed to leave the settings, etc alone, but the problem with that is the updates may not allow for an upgrade, the next choice would be to select a clean install, and when it gets to the blue screen select r to repair the current installation, but while this will wipe out all the updates which can be fixed, I am also worried that it will rewrite the master boot, to me it seems that there has to be away to reinstall the MCE portion without going through all that, I can see it on the disk just not sure how to make it happen! Thanks guys!