How do you intend to connect your computer to the Arduino?
Again many choices, such as serial, ethernet even optic fibre if you wish.
What operating system are you using on your computer, OSX, Linux or that other one which will remain nameless?
This may have some impact on determining how and what you use on the computer side.
Do you intend to program the software for your computer, say in some C stye or Java language, or using processing, google if you wish.
Does the program on the computer need to be time based of how does it operate.
Will the Arduino need to respond back to your program on the computer for any reason?
Do you really need the computer to control the Arduino, is there another way?
Alternatively, if you need the computer, do you really need an Arduino, there are other options too?
You see, knowing more specifically the criteria of your needs allows a faster and better response.
We are not able to read your mind and I try not to assume these days as much as I used to do
One thought that comes to mind is to check out the interesting 'Bitlash' suit from user 'billroy'. It may do exactly what you want.
Check it out here http://bitlash.net
I'm sure he will be about some time soon.