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As such, I advise you to first clear your "Cookies", "History" 
 and "Temporary Internet Files", thus making your computer 
 "fresh" in YouTube's eyes. My infamous "CHTIF" triple-cure 
 has resolved hundreds of YouTubers' problems. So why not 
 yours. Be sure to shutdown/reboot after doing this: 
 Internet Explorer 7: 
 ? Tools ? Internet Options ? Delete... 
 ? Delete files ? Yes 
 ? Delete cookies ? OK 
 ? Delete history ? OK 
 ? Close (browsing history window) 
 ? OK 
 Internet Explorer 8: 
 ? Tools ? Delete Browsing History 
 [. ] Preserve Favorites (leave blank) 
 [x] Temporary Internet files 
 [x] Cookies 
 [x] History 
 ? Delete 
Mozilla Firefox 3: 
 ? Tools ? Clear Private Data 
 [x] Browsing History 
 [x] Download History 
 [x] Saved Form and Search History 
 [x] Cache 
 [x] Cookies 
 ? Clear Private Data Now 
 Mozilla Firefox 3.5/3.6: 
 ? Tools ? Clear Recent History 
 Time range to clear: ? Everything 
 ? Details 
 [x] Browsing & Download History 
 [x] Saved Form and Search History 
 [x] Cookies 
 [x] Cache 
 ? Clear Now 
 
http://answers.yahoo.com/question/index?qid=20100324215208AA0KJ6i