I was looking at Mozillazine forums and apparently WMP10 is currently only for use in USA and is considered very 'buggy' at the moment. Advice would appear to be stay with version 9 until bugs in version 10 are ironed out.
Quote from there:
"Anyone with Windows XP in the UK will see a sentence in Windows Update saying: '(WMP10 update) is currently for USA users ONLY."
Hope this helps