In order to have XP visual styles inside Visual Studio.NET you can simply add a manifest file to devenv.exe. Simply download the file manifest.zip and extract it into the directory where devenv.exe resides. For Visual.Studio.NET 2003 it's: C:\Program Files\Microsoft Visual Studio .NET 2003\Common7\IDE

This will ensure that your forms designer uses the XP visual style (only if the FlatStyle of a control is set to System) and even custom-made addins are in XP style!