If you are using ASP.NET 2.0 CSS Friendly Control Adapters 1.0 with the Login controls of ASP.NET 2.0 you might have experienced some issue like multiple postback when using Internet Explorer. You might get a GREAT fix and explanation of the issue from Tana Isaac of Wellington, New Zealand.
Double Postback Problem - Cause (skip this if you just want the fix!):
A nice tool for Windows Vista, I will not tell you more you need to try it (especially you Didier ;-)
You can get it from this page: http://insentient.net/