Effectivement il semble qu'avec windows Vista, il y ai pour le moment des soucis, jusqu'à ce que la nouvelle mise à jour arrive il faut faire avec les moyens du bord.. donc, voici la solution en anglais mais pas trop le temps de faire là maintenant une traduction mais c'est pas sorcier..
A +++
Part I: Disable TCP Autotuning
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1. Click the Start (Windows Logo) icon
2. Select 'All Programs'
3. Select 'Accessories'
4. Right-Click on 'Command Prompt'
5. Click 'Run as Administrator'
6. A dialog box will ask you to confirm. Click the 'Continue' button to proceed.
7. Type: netsh interface tcp set global autotuning=disabled
8. Press 'Enter'
9. The message 'Ok.' should appear.
10. Type: exit
Part II: Empty your Java Cache
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12. The Java icon will appear in your task icon list in the lower-right hand corner of your screen
13. Right-click the Java (coffee-cup) icon and select 'Open Control Panel'
14. Under 'Temporary Internet Files', click 'Settings'
15. Click the 'Delete Files...' button
16. A window titled 'Delete Temporary Files' will appear. Make sure all options are checked and click 'OK'
17. Click 'OK' to exit the 'Temporary File Settings' window.
18. Click 'OK' to exit the Java Control Panel
Part III: Empty your

er's cache
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11. Clear your

er's cache.
12. Shut down all instances of your web

er.
That's all. You may now try to load the chat applet. If you have any questions regarding this procedure, please let us know.
This issue is likely to work itself out in the near future as future updates of Windows Vista and Java are released to the public -- once these updates have been released, your users will not be required to go through the above procedure.
How to Un-do the above steps
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If, for any reason at all, you need to undo the above procedure, follow these steps:
1. Click the Start (Windows Logo) icon
2. Select 'All Programs'
3. Select 'Accessories'
4. Right-Click on 'Command Prompt'
5. Click 'Run as Administrator'
6. A dialog box will ask you to confirm. Click the 'Continue' button to proceed.
7. Type: netsh interface tcp set global autotuninglevel=normal
8. Press 'Enter'
9. The message 'Ok.' should appear.
10. Type: exit