Uploading problems on Video sites using vista

Hey guys ,
Whenever I try to upload anything to a site, be it a file or a video, the progress bar instantly goes to 100% completed. Sometimes the upload will still continue, sometimes it will stop because the site thinks it has finished the upload. I try using different browsers but still the same problem .This is only happening on my Vista machine i tried uploading on xp and it worked but on vista it doesn't. So can anyone help me i cant upload .