There are a couple of possible problems you could be having.
1. Poor internet connection--Try using a wired connection, if possible, instead of a wireless connection to your router/residential gateway. A wired connection is not only faster but is also more stable.
2. Corrupted installation files--Delete your existing installation files and download the current 5.0.5 files that can be found here:
http://forum.us.runesofmagic.com/showthread.php?t=70835 If you don't have a FileFactory Premium account, use the DepositFiles or Frogster.de file links. (The files for the slim client are 5.0.0; they aren't and will not be updated to a higher release level than that.)
When you install and patch the game,
temporarily disable your anti-malware program, if one is installed. Also, if you use a 3rd-party firewall program, disable that one as well. (Windows Firewall is safe to keep running.) Once the game is installed and updated, re-enable your anti-malware program.
Hope this helps.