Check in the registry...
Start->Run->Regedit
Navigate to HKLM\Software\Microsoft\Windows\CurrentVersion\Uni nstaller
Perform a Find/Search from there for "SQL" and upon locating it - check the pane on the right for the UninstallString - that should be the command you need...otherwise,
Uninstall SQL
http://www.databasedesign-resource.com/uninstall-sql-server.html
http://support.microsoft.com/kb/290991
http://www.windowsitpro.com/Article/ArticleID/14070/14070.html
Uninstaller products:
http://www.ursoftware.com/
http://www.innovative-sol.com/uninstaller/