This has been discussed previously and there is a good fix in the wiki.
Here is the quick and dirty fix. I had a problem with USB comms between the UNO and Win 2000 machine. The program does not download correctly and the IDE reports an error.
Simple fix. Get USBSER.SYS that lives in \windows\system32\drivers from a WIN 7 machine and copy it into the same directory of your Win 2000 machine. Uninstall and reinstall the UNO 3 driver. Problem sorted.
Remember to save the old usbser.sys......