Hello Stanislav,
To uninstall SQL Server 2008, please see the instructions given on the following thread:
http://social.msdn.microsoft.com/Forums/en-US/sqlsetupandupgrade/thread/0998ebc9-f476-4788-998e-e051b0be1bef
If you need further assistance, just let me know.
Hope this helps.
Regards,
Alberto Morillo
SQLCoffee.com