Fried onboard USB ports?, "FF 301 keyboard error"

I have a Compaq Deskpro: P550, 768 MB (max), 7 GB hard drive, 2 USB
1.0 ports, XP Home. I also have a Sony Ericsson W800 camera/phone that
connects via USB to transfer pictures/music to/from. I've had the W800
about a year. At first the transfers went fine. Then more and more
frequently File Manager said there was no device connected. Usually
after several disconnects/reconnects the device came online. Last week
I couldn't get it to connect at all and in the process of trying, the
screen turned black except for "FF 301 keyboard error," and the system
hangs. As far as I've been able to find out, this error is thrown when
a key is stuck or for some other keyboard error. I have a new USB
keyboard, which hasn't given me any problem. I swapped USB ports to no
avail. I then removed the USB keyboard and pligged in a different PS/2
keyboard; the problem persisted so I don't think it's a keyboard
problem. Someone on one of the boards conjectured that the motherboard
may be fried. I don't know if he means that only the onboard USB
circuitry on the motherboard is fried or the entire motherboard is
gone.

Another helpful sole had a similar problem and put in a USB 2.0 PCI
board and the problem was solved. But I read a posting--somewhere--
that said something about 2.0 compatability between the PCI board and
the motherboard or certain features of USB 2.0 would not work. I'm not
a hardware guy; I have no idea what he's talking about. Since a USB
2.0 PCI board is cheap I thought it might be worth a shot. Replacing
the computer would be great but it's not in the cards right now. I
need advice. Thanks