100MB file upload? Sheesh, sounds like a courier with a CD would be quicker...
Check php.ini's values for upload_max_filesize and memory_limit; check max_execution_time as well - this has to be large enough that the script doesn't get bored and timeout partway through.
Consider providing the client with ftp access to a special upload directory - most web browsers (including the most common) are sufficiently familiar with this protocol.