- find the registry entry: [HKEY_CURRENT_USER]\Software\Microsoft\VisualStudio\7.1\Text Editor
- add a string with the name "Guides" and the value "RGB(128,0,0) 90"
- restart VisualStudio
This instruction works for version 2003, but by editing a similar registry entry it should work for versions 2002 and 2005 as well. The above gives a red line at column 90.