By processing the information that message provides you.
Are you running Windows Vista without the Platform Update (just Google it)? If not, you can scratch off that possibility. If so, just install the Platform Update (again, just Google it).
Are you making sure your graphics drivers are fully up to date? If not, get to it and update them.
If both of those have been eliminated as possible causes and you are still receiving this error then obviously, it is option #1 -- Your GPU does not support the functionality required by OBS. If that is the case, you will not be running OBS, period. Not until you get a graphics card (GPU) that does in fact support DirectX 10.
