The obvious option is to use a microcontroller to interface with your sensors. If you must use USB then there are microcontrollers with usb peripherals built in. Once you have your data from the sensors you can transfer the data to the .net application running on your computer.
Although if you asking such a question i think your going to struggle with a project like this. Personally i would concentrate on getting your sensors working with a suitable microcontroller, then once you know you can take measurements look into the data transmission side of things.
Having said this if you dont know how to program a micro controller that is another project in itself and might be worth spending time on before you embark on the hardware side of things.