Add vertical guidelines to Visual Studio by adding a simple registry key to this location:

[HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\7.1\Text Editor]

Now, add a string value named "Guides".

Set its value to something like RGB(128,0,0) 80, 100

The setting above will draw a Red guideline in Visual Studio at column 80 and 100. You can  have up to 13 guidelines. Adjust the values as you see fit.

Giving credit where credit is due, I believe Sara Ford first blogged about this about a year ago.