Darek,
The mouseover event is a client - side Browser DOM event, and therefore by
definition, it must be done with client-side script.
You can "inject" this script into the page from the server side with
RegisterClientScriptBlock, etc. but it still needs to be javascript since
the event of the mouse hovering over a control is a browser event and has
nothing at all to do with the server.
Hope that helps.
Peter

Signature
Co-founder, Eggheadcafe.com developer portal:
http://www.eggheadcafe.com
UnBlog:
http://petesbloggerama.blogspot.com
> Hi
>
[quoted text clipped - 6 lines]
>
> Darek
I think you can use addattribute with the javascript code on the server
side. Peter's solution works too