Are you using Vista / Win 7? If so, installing with UAC on and without right clicking the setup.exe and clicking "Run as an administrator" on the resulting dialog can cause a game to do a partial install. I also recommend installing in a directory outside of Program files.
First I would try this . . . right click the game shortcut, on the resulting dialog click Properites, click the Compatibility tab, check "Run this program as an administrator" . You can also set a compatibility mode on this tab as well. I usually start with none and, if needed, start with the newest mode and work backwards till I find newest mode that works best with the game. Try playing the game.
If that doesn't help, I suggest backing up any saves you have, uninstalling the game and reinstalling outside of Program Files as described below -
Backup your saves, uninstall the game, deleting any directories / files that are remaining after the uninstall.
Turn UAC to Never notify / Off depending on the OS.
Create a directory outside of Program Files, something like C:\Games. Right click the setup.exe and click "Run as an administrator" in the resulting dialog . On the first dialog of the GOG installer, click the Options button on the lower left corner and point the install to the Games directory.
After install, right click the game shortcut, on the resulting dialog click Properties, click the Compatibility tab, check "Run this program as an administrator" and uncheck the compatibility mode. You may need to use that but I would start without any compatibility mode selected.
Set UAC back to it's default setting. Try the game.
Hope this helps . . . =)

Thanks for your advice. I am running XP, had no problem with Myst. I do have the cds for both games, been thinking I might try installing them. Just afraid I could really have a problem after I do that. I know a "little" about pcs, not enough to be brave about changing things especially with the regedit. Thank you for your input. Mary Lou

Ok, thought it was the partial install from not installing as an admin problem . . . =)
You might read this thread for some tips on solving the problem.
I would start a help ticket with GOG support since the game should run great on XP.

Thanks for your advice, did as you suggested. Hope they can figure out what is wrong and give me some help. Thanks, Mary Lou

I just bought Riven yesterday and got the same error message. I wonder if this has anything to do with Norton blocking a virus at the time of install. Sent a message to support. Did you get an answer?
Thanks

This is definately the problem; i also recently purchased Riven. Norton reports that there is a virus in the menus.dll file and promptly quarantines the file basically crippling the whole install. Emailed gog support on the 29th of july still no reply..... had a similar issue with caesar iii think i will be putting on hold anymore purchases from gog until they can resolve this.

purchased riven today and had a similar experience. also have norton. funny though, havent had problems with any other games from GOG.

You can let Norton Know about false positives here;

submit.symantec.com/false_positive/

You can contact Cyan support here;

support.cyan.com/

Yes, but August 2010.