I was helping a colleague setup their new Windows 7 developer box today, and we couldn’t install SQL Server 2008. SQL Server’s installation would complain:
A previous release of Microsoft Visual Studio 2008 is installed on this computer. Upgrade Microsoft Visual Studio 2008 to the SP1 before installing SQL Server 2008.
Well, we’d already installed Visual Studio 2008 and Visual Studio 2008 SP1 so why wasn’t it working? Even after a reboot Visual Studio 2008 was reporting that SP1 was already installed.
In the end we had to install VS 2008 SP1 a second time, and after that the SQL Server install worked fine.
What went wrong? The developer had started to install SQL Server 2008 while VS 2008 SP1 was still being installed.
Matt's work blog 
If you are trying to install SQL-2008 and you also have visual studio 2008 installed with service pack one (SP1) and get the error
“A previous release of Microsoft Visual Studio 2008 is installed on this computer. Upgrade Microsoft Visual Studio 2008 to the SP1 before installing SQL Server 2008”
Then welcome to the Microsoft beta testing program even if you purchased the faulty software.
With XP the most common solution seem to be renaming the registry key
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\DevDiv\VS\Servicing\9.0
And replacing ‘9.0’ with ‘9.0Old’ however this does not solve the problem if you are using windows 7 and the only solution I managed to find that worked was to uninstall VS2008 and all the components and to then install SQL-2008 and finally reinstall VS2008.
its also work on Window 7 , i checked it