Windows 7

There are many reasons why an InvalidEnum may occur and the quickest way to understand the error is to look up the ailing function on the OpenGL reference pages.

Judging from opengl.org, the most common issue on Windows 7 is that users forget to install OpenGL drivers. Windows will install DirectX drivers automatically but will only emulate OpenGL over GDI or DirectX. However, this emulation layer supports only OpenGL 1.1 (GDI) or 1.4 (DirectX) and trying to use newer features, like VBO, will result in an error. Of course, drivers downloaded from the IHV's (nvidia.com, ati.com, intel.com) or from the manufacturer's website (dell, HP, etc) will bypass the emulator layer and provide better performance and features.

If possible, I would suggest checking GL.GetString(StringName.Vendor) and displaying a warning if it contains "Microsoft Corporation". You should also check that all necessary extensions are available before trying to use them (for example, VBOs require ARB_vertex_buffer_object or GL version 1.5, FBOs require EXT_framebuffer_object or GL version 3.0 and so on.)

Note that OpenTK 0.9.8 has not been tested on Windows 7 at all. It should work, but the first version to officially support Windows 7 is 1.0-beta-2. If possible, I would suggest upgrading to the latest release.

In that case, the error is right: the correct method is GL.EnableClientState(ArrayCap.VertexArray). Personally, I would suggest avoiding vertex arrays completely: vertex buffer objects are faster and safer to use under .Net. (The issue with VAs is that the data store is your responsibility, which means you have to guard it against the garbage collector. VBOs store the data in video memory, which is both faster and safer.)

In OpenGL 1.x, 2.x and 3.x with the compatibility profile, you can check for extension support using:

string[] extensions = GL.GetString(StringName.Extensions).Split(' ');
if (Array.IndexOf(ext, "GL_ARB_vertex_buffer_object") != -1)
{
    // ARB VBOs are supported
}

Moreover, every version of OpenGL moves a number of extensions into core. For example, OpenGL 1.5 introduced vertex buffer objects:

float version = Single.Parse(
    GL.GetString(StringName.Version).Substring(0, 3),
    System.Globalization.CultureInfo.InvariantCulture);
if (version >= 1.5)
{
    // VBOs are supported in core. No need to check for "GL_ARB_vertex_buffer_object"
}

As a rule of thumb, core functions reside in GL., while extension functions live in GL.Arb., GL.Ext., GL.NV., etc. However, some extensions may modify the behavior of core functions (for example, ARB_vertex_buffer_object changes how GL.BufferData works when a VBO is bound - read the extension spec for more information).