Why am I getting a scroll bar at the bottom of my posts
What is your screen resolution?
Right-click on your desktop and then click on Properties (or Personalize if you are using Windows Vista). Open Screen Settings and you should be able to see your screen resolution.
So long as the width is 1024 or greater (common size with that width is 1024 x 768) you shouldn't have a horizontal scroll bar.
If however you are still using 800 x 600 screen resolution (or any other weird ratio with a width of 800 pixels) then you may have a problem. 800 pixels width screens went out with the ark and most websites these days are optimised for a minimum of 1024 pixels width. I don't know if that is the case with this group, but as Simple Machines Forums are fairly new (they've been around for just over two years) I imagine they may not work too well with 800 pixels width screens.