Myst V uses SecuROM copy protection. What you're getting sounds like a generic SecuROM error, which may occur if:
-- a. You aren't logged into Windows with full Administrator privileges when starting EoA and/or didn't install the game while logged in with full Administrator privileges.
-- b. You have software that installs virtual disks on bootup.
-- c. You're trying to install and play EoA over a network. Local machine only!
1. Using Device Manager (right-click on My Computer icon > Properties > Hardware > Device Manager), remove all virtual disks installed by CD/DVD burning or copying programs. For example, Daemon Tools, Alcohol 120%, CloneCD, Easy CD/DVD Creator, Nero Burning ROM, etc.
-- a. Do NOT reboot after removing the virtual disk(s), before starting EoA.
-- b. You may need to prevent the program(s) from loading on bootup, using the System Configuration Utility, before starting EoA.
----- 1. Click on Start > Run.
----- 2. Type msconfig in the Open: box, then click on OK to run the program.
----- 3. Click on the Startup tab.
----- 4. Scroll down to the program you want to prevent from loading on startup, and click on the check box to clear it.
----- 5. Click on Apply, then on OK.
----- 61. Reboot, then start EoA.
-- c. Worst case, you may need to uninstall the program itself and reboot, then start EoA.
2. Delete your SecuROM folders. If you're NOT using an update service like Steam (for Valve apps) that uses SecuROM, delete the SecuROM folders under \Windows\Application Data\SecuRom\UserData or \Documents and Settings/<username>\Application Data\SecuRom\UserData. If you ARE using an update service, this will probably kill it, so try renaming the SecuROM folders instead of deleting them.
-- a. Since \UserData has a hidden file, set Windows Explorer to view hidden files. Click on the View tab > Folder Options > View > Files and Folders, Hidden files > Show all files, then click on Apply > OK.
-- b. The files in \UserData are also read-only, which you may need to change. Right-click on a file, select Properties in the resulting drop-down menu, and clear the Read-only check box.