While looking around for a new keyboard I figured out what this problem is called. Its called Keyboard Ghosting. A lot of keyboards do this but on less used or different keys. If you look at specifications for keyboards normally its listed and says something like this "Multi-key anti-ghosting ensures the most accurate simultaneous key presses". The Thermaltake keyboard doesn't say anything like that. This appears to be the reason why this keyboard has the problem.
I had Newegg replace the keyboard and the new one does the exact same thing. I assume its a keyboard build problem since it has happened on both keyboards I've tried. If I was buying a normal keyboard this might not have been an issue but when you buy a gaming keyboard and your game requires that you use those keys for moving your character things get complicated. This might just stop me from ever picking a Thermaltake keyboard unless you can tell me its not a problem with your other types of keyboards. I'll check back in a week to see if there is any response.
I am using a Thermaltake Challenger Prime keyboard. When I am playing games if I hold CTRL down and try to move with specific arrow keys it wont work. Pressing BACK and LEFT arrow keys or BACK and RIGHT, it will only do one or the other. Pressing FORWARD and LEFT or RIGHT works fine. I saw on the forums someone else had trouble similar and you suggested trying another computer so I took it over to my parent's place and tried it on their machine and has same trouble.