I have all the Myst games (including all three versions of the original game0 plus 5 separate installs of URU offline and online with no issues. I'm Cyan took this into account when designing Uru Live so I dont anticipate any issues. I n addition if there where any issues with installs of other Myst games, I would expect that would be stated on the download page for the Beta Installer. So really can't see this being an issue.
With regard to editing your registry there is a very simple way to protect your self. Make a copy before you edit it, That way if anything goes wrong you can revert to the saved version. THough in the case of multiple Uru the changes are trivial and the worst you can do is currupt your install which can be corrected by a re-install.
On the subject of downloading zips and compromising computer security, I know of now case of a computer being infected with anything from just downloading a zip. THere are some very simple precautions which can be takrn to protect yourself when it comes to unzipping them.
1. Check the bono vides of the source
2. Set a restore point before unzipping them
3, Use Winzip to unzip them (it has an option to virius scan the files before you unzip them)
4. Unzip into there own directory.
5, Dont run .exe files included especially if they claim just to be documents (unless of course you want to install the program)
6. If anything untoward happens restore your system using the restore point prior to unzipping the files
Follow the above and your chances of damaging your system is minimal.
To be absolutely sure invest in an external disk big enough to back-up your system to. Make an image of your C drive, Then in the case of a complete disaster you can just restore the entire system.
Another way to protect yourself which also works is to cretae a Windows Virtual Machine using one of the commercially available VM packages. An unzip and install suspect packages in that enviroment. Then if the package contains any viriuses of malware etc, Its the virtual machine that gets infected, so all you need to do is delete the file which contains the suspect package and start over. If the programme proves to be safe you can then install it in your real windows. Virtual Machines are designed for amongst other things test code on without causing a threat to the host system, there extensively used in industry for testing as you can completely corrupt them and still be back up and running within an hour. I can reccomend VMWare and whats better its free and industry standard. I currently have an install of Linux and Windows Vista both running on Virtual machines on my PC both hosted by Windows XP. Anythiong I install in Vista can only write to the registry of the windows installed in the VM machine, there is no way anything can be installed to the host systems register because it cant be seen by installers.