Well, are you using Vista? The same thing happened to me on my old notebook, whenever I was connected to the internet via WLAN in Vista. My solution: no WLAN when playing RO.

Anyway, I googled a bit how to solve this problem, because it seems there are other people experiencing the same problems (not only in RO, but in Windows itself when using Windows Media Player). Someone suggested to update the WLAN drivers, another one recommended to buy a new WLAN adapter, dunno. @.@
Btw, is it only the login screen, or is it the same ingame?`For me, the "bug" happened ingame while playing as well, always making my game slower, thus lagging every minute, it was quite annoying. :/
If you're not using Vista, disregard this. ^^'