Hello imran,
SQL2008 CTP 5 and priors require the uninstallation of SQL Server Management Studio 2005 if exists one which is installed earlier.
This means SQL2008 SSMS and SQL2005 SSMS are not supported to run side-by-side.
But however I managed to install and run both SSMS's for 2005 and 2008 CTP6 by changing the installation path of Client Tools of CTP 6 from the default installation path.
I used C:\Program Files\Microsoft SQL Server CTP6\ for CTP6 installation.
You can read my blog post on this topic at Installing the Microsoft SQL Server 2008 (Katmai) CTP6
Eralper
SQL Server Programming