You're re-installing from..?

Did you take a brand-new rohan client download from the mirrors provided on the website - anytime recently? Aka, within the past 2 months?
How many times did you download it (i.e.: have you tried (a) different mirror(s)to see if you got the same problem)?

I'm asking this, because your client could possibly be out-dated if you're an old player that just wants to restart again using the old client.

Apparently renaming the rohanclient.exe does no longer work so it seems you can skip that one. Other than that i can only suggest to make sure your virusscans/anti-malware software/spybots/firewalls are DISABLED whilst installing (and running the game the first time). Not just "offline", but totally disabled. Make sure that while doing so, you're not browsing any suspicious websites (or none at all).
Disable or lower the security on your webbrowser as well and make sure that you install and run as "as administrator". Run the game through Loader.exe (and not through rohanclient.exe - or preferably, just use the generated shortcut to desktop)