This message poped up when i was installing burning crusade on the 4th disc, and you can only click ok making the installation to quit, i read around on forums on the internet finding many solutions that doesnt work, but i just wanted to share with those who havent tried this...hehe
when the message came up for the second try of installation i began to search the web for that message without clicking on the "ok" button. and what happened was that it seems the installation continues in the background so all you have to do is wait, maybe it doesnt work for everyone but it did for me but i never would have guessed that cause you cant se any progression on the installation.
so at least just try to wait if it happens for quiet a while and see if you are as lucky as i am.
just wanted to share this solution cause it might just be that easy.
Just use your WOTLK disc. It will update any WoW installation to whatever patch it was at the beginning of the expansion.
Either that or just download the client from the website. Either way your going to be downloading about 5 gigs worth of stuff anyway.