I had the precise problem. It occurred immediately after I used Windows Update to run Dot-Net 3.0 Sp1. That update failed, so I left it. The next time I fired up QB 2008, I got the loading screen, then nothing. I couldn't wait for a fix, so I bit my tongue, paid $300 for a one year maintenance contract,and called for help. The tech (a very nice young man in Manila) and I spent about three hours doing a number of things: running the QB repair utility, a webpatch, uninstalling and reinstalling QB twice, attempting (unsuccessfully) to remove Dot-Net 3.0, and some other steps I can't recall.
In the end we got it working. Since the fix, I note that each time I load QB, it starts with a dialog box to the effect that there is an update waiting. But if I select "Update", the update screen shows that there's nothing to download.
I'm convinced there is a bug which QB will eventually fix. For now, I feel your pain.