Go into your registry and add these lines (if not already there)
[HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Valve\Stea m\Apps\20920]
"DirectX"=dword:00000001
"VCRedist"=dword:00000001
"dotNetFx40"=dword:00000001
For the ones who are not very computer capable, follow these instructions (this is all under windows 7, other windows-based OS's should be similar in commands)
1) Open the start menu, type in 'regedit' hit enter (without the quotes)
2) Navigate to [HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Valve\Stea m\Apps\20920]
3) In that 'folder' there may be just a 'directx' value (or may not be) and at the very least, a (default) value.
4) Inside that folder, on the right hand 'screen' of the two, right click and choose: new--DWORD
5) Name that new dword 'DirectX' (if applicable), then double click the name and change the value data to 1. (without the quotes around DirectX)
Do steps 4 and 5 for the following two DWORDS: 'VCRedist' and 'dotNetFx40' (without the quotes around the values)
This should get people up and running.
EDIT: I feel I should also add that I'm running my OS with full admin rights and UAC is turned off, if UAC is on, you may need to right click 'regedit.exe' in the search bar and choose 'run as administrator'