I lost the squiggly red underline feature in Visual Studio 2010. I experimented with some plugins/extensions like Coderush/Code map. I have reset all settings for fonts and colo
in javascript I solved this by going to Tools > Options > TextEditor > Javascript > advanced > Uncheck Show Syntax Errors.
If your want to add/remove C/C++ underline squiggles, try this:
Add-ins often turn off the Visual Studio features so they can replace them. And of course don't restore them when you uninstall them, so you're pretty much forced to keep using them if you can't figure out how to undo the setting changes.
Tools + Import and Export, Reset might be your best shot. As long as you haven't customized VS yourself.
The specific setting is Tools + Options, Text Editor, C#, Advanced, "Underline errors in editor" check box.