I'd say one or more of the files is corrupted during download. It happens.
You might try going here, instead:
http://forum.us.runesofmagic.com/showthread.php?t=66641 You'll find two lists of direct download files, one for the Full client, one for the Slim client.
Pick a list, then download no more than two files at a time to keep your download speed reasonable. When done, run the ROMSetup.exe file.
Be aware that after installing the game, there will be a dozen or so updates that will need to be downloaded and installed the first time you run the game launcher. I'd recommend temporarily disabling any a/v program that's running for both the game file downloads and the patching process. You can re-enable it after you're done.