I ran your VI and saw the symptoms you describe. I'm also running Win2k and LabVIEW 6.1.
I deleted your OK button and added a new one (not just right-click >> Replace), created a new property node, and edited the event structure (to recreate your original) and the problem went away.
Did you customize the OK button? Something strange may have happened in the control editor. You can change the text and size of the button without using the control editor.
Try deleting the button and adding a new one.