You might have a corrupted installation file causing this.
While I know that it takes a long time to download the game files, you might try deleting the installation files that you have, and go here:
http://forum.us.runesofmagic.com/showthread.php?t=66641
These are the alternate, direct file downloads for both the Full and Slim clients. To keep your downloading speed maximized as much as possible, I'd recommend downloading no more than two of the files at a time. Don't mix and match them--each set of files goes together.
Hope this helps.