Okay, seriously, I need to say something, because I doubt most of the people commenting in this thread have ever dealt with either health IT, healthcare software, or any of the related nonsense.
There are kinda four flavors of machine setup I ran into while in that field: big server banks for on-site hosting (think huge enterprise VM farms, for data warehousing and record storage and virtual desktop hosting), care provider systems (think like tablets, doctor office computers, nurse workstations, room workstations), cart computers (used for things like running the sonogram or cardiogram equipment, or for other studies), and actual integrated devices (for, say, data collection).
The care provider systems are usually comically locked-down, tablets and phones having the meanest management software they can (no apps, limited connectivity, remote wiping, and so forth). Workstations tend to be centrally managed, have images pushed regularly (ha!), and often use AD and smartcards to handle authentication. One place I've seen took this a step further, and basically just booted users directly into a VM hosted on the server farms mentioned earlier. You can't use USB devices, you have highly-regulated clipboard access, and so forth--this is done to prevent HIPAA breaches. Which is kinda silly given other workarounds, but whatever makes people feel safe and the CIO happy. These workstations run some enterprise version of Windows, probably 7 Pro. Those silly-long extended service agreements you see on Microsoft? Hospitals are some of the people keeping that alive, and they will pay obnoxious amounts of money for the privilege.
The cart computers are typically like the workstations in terms of functionality, but they may have software specific to the device they're talking to. They might not be as locked down (e.g., only acting as thin clients to a remote VM), but they are still running Windows.
The device computers may run some kind of RTOS. In some cases, they'll be running a customized Windows CE installation--which is totally reasonable. There are a lot of good guarantees that that can give a development shop, least of all that they can call up Microsoft instead of StackOverflow and say "Hey, this function does x, it's documented as y, and we're paying you a lot of money, so what the fuck?". Windows Embedded (which is I think the successor, am not sure).
In all of these cases, Windows itself works pretty damned well.
It runs the software everybody needs, it has the enterprise deployment stuff figured out through decades of improvement, and really there is no reason to be scoffing at its choice.
Now, if folks have goofed up and thrown a stupid AV policy on the machine, that's a different question entirely. Health IT is full to the brim of people basically just punching a clock and being unable to get anything done in a reasonable amount of time. Sometimes, they do awesome things, but mainly they are just custodians standing between doctors and really really stupid policy decisions that seemed good at the time.
EDIT: Removed unrelated example at top.