Looking for a Development Platform to be Used on Windows

Something like this?

http://us.brainboxes.com/faq/items/how-do-i-install-my-brainboxes-ethernet-to-serial-device

It would be helpful to identify the instrument you just acquired.

You need to be a little more specific. Ethernet is the hardware. What communication protocol(s) does the instrument support? For example, Modbus TCP/IP is one commonly used in industrial applications.

The first application development platform I always looked at for Windows apps is Microsoft's Visual Studio. It supports a broad variety of languages for various applications and networks.

Hooker

Visual Studio would also be my first suggestion if you are confining yourself to the Windows platform. My second suggestion, if there is any possibility for extending to the Mac, is to look at RAD Studio from Embarcadero

Without more info about protocols and ports it is hard to guess. However, it may already have software delivered on a webpage if it "speaks ethernet".

Perhaps all you need is a browser or perhaps a browser with a list of functions to send in a local html file.

What open ports does it have. Does it use DHCP and setup a TCP/IP address or does it use some other protocol?

If it uses TCP/IP then it should have an open (listening) port that can be found with a scanner like wireshark or nmap. If so, then a browser with a html page full of functions to send to that port should work. Chances are that if it is TCP/IP then you should be able to see a page at port 80 or 8080 already and you may not need to write anything.