I am trying to figure out a way to monitor the level of liquid in a large fixed container and output that information (via serial, USB, Cat5, etc.) to a program on a windows machine to display the current levels.

I have been looking at different "off the shelves fuel lever sensors"  that will probably work to measure the level of fluid, but I am unsure how to take that signal and convert it to something that is readable on a users PC.

Tons of COTS units will take care of all of that for you...(Instrument Manifolds) just search on data acquisition units.  A few hundred clams should be enough to buy something simple, a few hundred more for a high-quality sensor (depending upon your material).

You need to give some serious consideration to your level instrumentation for a "fuel" sensor.  Instruments that are rated for presence in a fuel tank are industrial instruments rated for what is likely a hazardous area.

Are there other characteristics of the fuel, besides flammability, that need to be considered?  Corrosive, erosive, nasty, sticky, gooey, a fume generator or foamy?  

And you should be aware that the selection of level instrumentaion is governed in large part by available access to the tank.  Access from the top?  from the side?  from the bottom?   There are serious considerations to poking holes and welding fittings into tanks and vessels, particularly fuel tanks.

Another consideration is how close the tank is to where you want the data.  Industrial level sensors typically use a 4-20mA analog signal that can travel a mile through copper wire, but RS-232 has what, a 50'-60' limitation?  

There are converter modules that will convert anaalog 4-20mA to serial or any of several industrial protocols, like Modbus,which is a serial protocol that runs on RS-232, RS-485, and nowadays, I suppose Modbus TCP on ethernet.  Google: "converters 4-20ma to serial" for numerous options.

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