A couple of guys using Vista have told us that the problem is that openlp.org is looking for a file that doesn't exist in DirectX 10 anymore (openlp.org uses DirectX 9 headers). You need to download and install D3DRM.dll. You should be able to simply copy the file into the openlp.org directory (on most systems it should be "C:\Program Files\openlp.org") and openlp.org should work fine. Let me know about this, I'm operating on hearsay.
On the other hand, I've moved to Linux full time now, Kubuntu 7.04 (Feisty Fawn), as my main desktop, although I'll still have a Windows box around to develop openlp.org. If you haven't looked at Linux yet, I would strongly recommend you do! Download a CD image from www.ubuntu.com and try it out!