If you are trying to install SQL Server 2005 client tools and continuously getting failure message because of MSXML 6 installation which already exists on your machine then it may be due to the corrupted installation of MSXML 6 which blocks the installation of client tools. You can fix it as follows:
Go to Control Panel-> Administrative Tools->Add Remove Programs and try to remove it. If it removes smoothly then just install the Client Tools as before and you are done.
OR
If you found that you are unable to uninstall the MSXML 6 because of some wiered error then you need a tool called Windows Install Clean up from here
Donwload it and remove the corrupt installation of MSXML 6. Once removed, you can now install the Client tools as usual.