im using presentation software on my pc which is connected to a projecter to add effects to my live video, our cameras are connected to a hardware video switcher, and i need to receive that live video feed to my pc so i can add effects to it and output to my projector, so i need to know to receive that live video feed.
You're doing it wrong. Key from your effects source on to your live video with the switcher. You will get much lower latency and better performance. Alpha keying is ideal if your switcher supports it, but depending on what you are doing, luma keying (or less likely chroma keying) may also work.
There are a select few situations that may actually merit running the video through the computer as a filter, but they are pretty few and far between and most likely don't apply here.