Computer Loses Disk Drives

My Dell 8600 with WinXP SP3 loses track of its disk drives, both internal and external. Sometimes it forgets it has internal DVD reader, internal CDRW, external USB/Firewire 10GB, external WD 320GB USB, external USB connection to my wife's Nextar portable photo frame (which is supposed to act like a disk drive). I also have a USB connection to an APC power supply backup, which it usually doesn't detect either. Fortunately, it usually can see my WD 230GB USB (connected to a computer USB port), since that is my main "home" data storage. The WD320 and APC and HP scanner are connected via a USB 2.0 hub. The photo frame was connected directly to a USB on the front of the machine.

What is also frustrating is that it doesn't always lose the same devices. Sometimes even the internal disks are "lost" until I reboot, at which time it's likely to lose something else. The internal drives came with the unit, untouched. I also have a Pinnacle Studio 8 card, which I have tried to use due its firewire port. As with the USB, sometimes it works, sometimes not.

Any ideas?
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:confused