I bought Magicka via Steam this evening, and just installed it now. It demanded to repair my XNA install, so I let it.
I'm trying to run it, but it's just quitting as soon as it opens.
If it asked to repair the XNA framework on your first start it most likely means that it was already installed. The other dependencies should be the .NET framework, either 3.0 or 3.5, and DirectX. I would try to install all of that manually and see if that fixes it.
Just to be sure also update your graphics card drivers.
It had this exact problem with fresh Windows 7 install. The game seemed to boot up fine, smoothly progressing throught slash screens but the game crashed as soon as the book came into view.
The way I solved it was to install the A.R.E.S. demo. They both use XNA and A.R.E.S. doesn't have the XNA bug in the installer.
The only other game I had installed at the time was Left 4 Dead 2. But I'm pretty sure the L4D2 didn't effect Magicka.
Ah! The magic that seems to have finally made a difference was manually setting the system to the graphics driver. (My system has Optimus tech. It works well, when it works, but a lot of programs are missing from its application list.
On my machine: NVIDIA Control Panel, "Manage 3D settings" from the treeview on the left, and then go to the "Program Settings" tab and click "Add" to get an 'open file' box. Locate Magicka in your file directory structure, and then change the preferred graphics processor in the dropdown on the NVIDIA control panel. Then don't forget to click "Apply"! It doesn't auto-save.