+2  A: 

Absolutely. The engines are isolated from each other, so, you can run both in parallel or remove/uninstall components if you need.

Hope this helps!

Eduardo Cobuci
A: 

That should be safe.

What we have had problems with is: if we install SQL Express first, then the management console will not install, when you install SQL Server.

Shiraz Bhaiji
A: 

I had kitten parts splattered everywhere when I tried to do exactly what you are asking about. So I burned it all down and reinstalled, installing SQL Server 2008 first, and then making sure to de-select the database components in the Visual Studio installations.

So far so good, but my SQL Server installation is used sparingly.

John Clark
I'm sure you had the kitten parts, but I uninstalled SQL 2005 and then installed 2008 and so far, all kittens are safe and sound.
J. Pablo Fernández