If it says wrong username or password I'm not sure you will get much farther by installing another version of the game, however.
User names should contain only letters from a-z in both passwords and usernames to be on the safe side.
Password is capital sensitive so you must keep that in mind.
If you have numbers in your password you cannot use the numpad, only the numbers above the letters work.
Extremely long usernames can sometimes be an issue as well.
If nothing helps, try looking in the big help thread on the main help board. And if still no luck, try creating a new account with another password/username, staying away from symbols and numbers.