Long shot here, but make sure you're running the latest version of Active Sync (4.5) or if you're on Vista, download the latest Windows Mobile Device Center. And for good measure, and this (sometimes) will cause errors like what you're having, you must make sure you're running .NET 2.0, better yet .NET 3.5 and all updates for .NET before attempting to flash.
My sister was getting everything from the errors you have, to blue screens of death mid-flash (YIKES!) because of outdated, and missing .NET. Thank god the BSOD didn't kill the phone!
