Case 1. If you have not installed Visual studio 2008
Step 1. Install .Net FrameWork 3.5 SP1 On Windows 7
Step 2. Download Microsoft SQL Server 2008 Management Studio Express
Step 3. Proceed to installation as mentioned later in this post.
If you get error shown below during installation, then you might have visual studio 2008 or some of it's component installed on your system which you can check by opening control panel > programs.
Case 2. If you have Visual studio 2008 installed
If you have Visual Studio 2008 or some of it's component installed as shown in above image follow steps mentioned below.
Continue to step 1 and 2 mentioned above
Step 3. Download Visual Studio 2008 Service Pack 1 either as online installer or as ISO image
Step 4. Install Visual Studio 2008 SP1
Hope this helps