Software like this invariably uses a technique called TCP stack fingerprinting to determine whether your device is of the sort that requires the software installed. Basically, invalid or strange TCP packets are sent to you upon first appearance (or at DHCP time or something), and the response to each helps the security system to decide whether you're a Windows box, a Linux box, a handheld something, or a game console, because the stack on each of these systems responds a little differently to out-of-RFC TCP junk.

There are several pieces of software out there, most notably OSfuscate (http://www.irongeek.com/i.php?page=security/osfuscate-change-your-windows-os-tcp-ip-fingerprint-to-confuse-p0f-networkminer-ettercap-nmap-and-other-os-detection-tools) and sec_cloak (http://www.hacker-soft.net/Soft/Soft_2304.htm, but the link is quite broken), that reconfigure your Windows TCP stack via the registry to appear to these tools like something entirely different. After doing that, just tell your IT department that you need to get your other device on their network and most places will whitelist you. The most popular choice for what to emulate is a Sega Dreamcast; why that is the case is left as an excercise to the reader...

At most places, looking like something that can't run their spyware gets you online, but some places want to see the hardware (especially for game consoles), so if you're concerned, say the machine runs Linux sometimes and show it to them running Linux (off a LiveCD if you must) if they ask. Then use software to make your Windows look like Linux too, and the exception they'll have put in for "a Linux box with MAC xx:xx:xx:xx:xx" will cover both systems.