Just wanted to share this with any of you having the same pain.
If you are using TestDriven.NET and/or NCover on Win XP 64, and get this error in the event log:
.NET Runtime version 2.0.50727.42 - Failed to CoCreate profiler.
For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.
Along with "Failed to CoCreate Profiler" in the VS2005 output window, when trying to "Test with.. coverage", the best fix I have at the moment appears to be to set your test assembly (e.g. Project.Test, rather than the actual project itself) to be a 32bit assembly, via the Properties->Build section. This seems to be a bug in NCover, and will hopefully go away soon.
Tim Anderson talks a little about this here, and this link provides some more technical info.