(I understand that you already upgraded your database, for what you say, from 1.9 to 2.0...)
mmm, I would say: try settnig the file gallery to set files on disk instead of in database.
And alternatively, try doing that upload using the TikiLiveCD. It might be that you have a problem due to some missconfiguration of your xampp on windows. If you cannot reproduce your problem on the TikiLiveCD, then keepp searching on your xampp, not on Tiki.
My 2 cents.