I'm using my ATI cards software to rotate the picture within windows xp.
If you are using rotation software with the video card, you may be able to use other frontends as well, that do not support it natively.
...Keep in mind there are some video cards that don't work well with mame with windows being rotated. I
think it has something to do with the windows rotation software and mame hardware stretching trying to access DirectDraw at the same time.
You may be fine ..But If you have problems with
some games (as I and others have) I recommend that you don't rotate windows and use a FE that can be orientated vertically and just set the FE to launch on start-up.