Multithreading - Single WinForm Displaying Two TextBoxes

Here's the setup: I've created a class that represents a temperature and humidity monitor. This class handles all of its own operations (connects to the monitors via TCP, reads the temperature and humidity values at set intervals, logs the data, sends email notifications, etc.). I've got a single Windows form that has two textboxes for displaying the values of temp and humidity. When I start the application, I simply instantiate an object of this class and let it do its thing. Every five minutes, it opens up a socket connection, pulls in the data I want, sets the Temp and Humidity properties of the object to the values it obtained, and fires a DataUpdate event which is captured in an event handler in the Windows form.

When this event occurs, the textboxes on the form are updated with the values from the Temp and Humidity properties of the object. Works like a champ. However, I've been playing around with multithreading. Since a disconnected device will cause my entire application to block when it tries to connect, I decided to place the socket connection/data acquisition method into a separate thread. If it can't find the device, no big deal -- at least it doesn't lock up my UI. This thread is instantiated and started inside the object code, NOT the Windows form.

Here's my question, the socket connection/data acquisition method is run inside this new thread and sets the properties of the object when it's done. So, why do I get an illegal cross-thread exception on my Windows form, now, when I try to update the textboxes? It seems to me that the instance of the monitor class is running in the same thread as the Windows form. I only employ a separate thread when I want to run the socket connection/data acquisition method. Is my understanding flawed? I can display the updated property values in message boxes that are called from the Windows form. I just can't manipulate the textboxes.