Heya Universeman,
At one time I had the same exact problem with FS 2004. Three things that I can recommend to resolve this issue:
1.) Since you are running Windows XP make sure that you have the SP2 upgrade installed. This upgrade fixes some Graphics Card display issues. Also the Windows .Net Framework is necessary to install and use some drivers. You can get both of these at Microsoft's website if you don't have them.
2.) Go to
http://guru3d.com/ or
http://www.ati.com/ download and install the latest Graphics Card drivers. What you probably want is the latest version ATI Radeon Catalyst 5.10 Windows XP driver. To my knowledge, this driver is compatible with all ATI hardware.
3.) Using Windows Explorer, locate the FS9.cfg file. First open Windows Explorer and at the top menu bar click Tools --> Folder Options then click the View Tab. Look for a radio circle button that says "Show Hidden Files and Folders." Click to select this option then click OK.
Now open C:\Documents and Settings\(~your Name)\Application Data\Microsoft\FS9 and look for the FS9.CFG file. It is very important to first make a backup copy of this file! Next look in the text and see if there is a section like this:
[DISPLAY.Device.ATI Radeon 9000 .0]
Mode=1024x768x32
If you have a generic display device listed then change it to the above specific Graphics Card. If you don't have this entry at all then Copy/Paste this at the end of your FS9.cfg file. Save the FS9.cfg and then exit.
These tweaks will not only rid you of that annoying blue line but also improve your framerates.