In some forums in found that people asking whether he/she can install Visual Studio 2010 side-by-side with earlier version(s) of it.
The answer is YES.
I have Visual Studio 2005, Visual Studio 2008, and Visual Studio 2010 installed in one machine (OS is Vista Ultimate 32bit).
If I open Program Files, I can see that they are installed in different folders.
So you don’t need to worry to install Visual Studio 2010 in the same machine having previous version installed.
Even though you can install in random of back-ward order, I suggest to install in forward order, VS2005 -> VS2008 -> VS2010.
Having more than one version of visual studio installed is useful if you’re working with a lot of projects built using different version of Visual Studio. Because although VS2010 provides multi-targeting feature for the framework, the Project and Solution is not compatible and sometime upgrading is not a good solution if your client still using VS2005 or VS2008.
Hope this information useful for you.