You may have to right-click on the Runes of Magic icon that's on your Desktop and select "Run as Administrator" from the context menu. This sometimes happens, especially if you're logged into the computer on a "Standard" (non-Administrator) account.
Failing that, you can go here:
http://forum.us.runesofmagic.com/showthread.php?t=31461
Read Tigsman's starter post and his instructions on how to download and install patches manually. Then, beginning with "patch_5.0.0.2538.en_5.0.0.2539.en.exe" and working your way upwards, download the manual patches.
Copy them to your main Runes of Magic game folder, then run them in
ascending order, oldest to newest.
Let me know if either of those suggestions work for you.