I've been having the same problem that so many of you have - my USB devices are suddenly "not recognized" or reported as "malfunctioning." I have an iMac with the Intel processor, running Leopard on one side of a partitioned drive (this is also known as "the side that works") and Vista on the other side. Having been raised to be polite, I won't say what I call that side. I have an HP LaserJet 4100 as my workhorse black-and-white printer, an HP Photosmart 8450 to print photos, an HP ScanJet 4070 scanner, and a Maxtor external drive attached to a USB hub. Although everything seemed to work together quite well to begin with, recently I've been having constant problems with my USB devices. Since they work reliably on the Mac side, this has to be a Windows problem in my case. I noticed that the problem frequently occurred after the computer had gone to sleep and been awakened, so I turned off "sleep" and that seemed to help, but it didn't cure the problem completely. I still have trouble sometimes after I've re-started the Windows side. I have read all of the posts on this topic and tried your suggestions. For what it's worth, the only thing that seems to work reliably for me is:
1. Shut everything down.
2. Cut the power to all devices (which I do by shutting off the battery back-up to which all of my devices are connected, but unplugging them also works).
3. Disconnect the USB hub from the back of the computer.
4. Wait a minute or so, and then turn the power back on.
5. Re-start Windows, and WAIT until it is completely operational. I find that this is a good time to spend a minute or two contemplating the utter injustice of having spent good money to acquire the hunk of junk that is Vista.
6. Cross your fingers, and plug the USB hub back in.
This seems to reset everything that needs to be reset after Vista does whatever damage it does. I hope it helps someone else. If any of you who know more than I do can figure this out and come up with a way to prevent this problem entirely, my blessings on you and all of your descendants.