Вот подробный ответ
(Может уже и поздно
To make your Windows Forms application support Windows XP visual styles, be sure to set the FlatStyle property of your controls to FlatStyle.System and include a manifest with your executable. A manifest is an XML file that is included either as a resource within your application executable or as a seperate file that resides in the same directory as the executable file. For an example of a manifest, see the Example section for the FlatStyle enumeration. For more information about using the visual styles available in Windows XP, see the Using Windows XP Visual Styles in the User Interface Design and Development section of the MSDN Library.
А вот пример нужный manifest
The following is an example of a manifest to allow your application to support the visual styles available in Windows XP. The manifest can be added to your application as a resource or as a stand-alone text file. The manifest file resides in the same directory as the executable file, and the name of the text file must be the complete name of the executable file with an additional file name extension of .manifest. For example, the manifest file for MyApp.exe would be MyApp.exe.manifest. This example assumes that the FlatStyle properties of the controls in your application are set to FlatStyle.System.
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
<description>Your application description here.</description>