This question is not specific to TestComplete, but I thought someone on here might know. Our application has a WPF user interface. We have been running TestComplete on Windows XP and Windows Vista, ...
As far as I know, WPF has its own mechanism of drawing text. Probably, this depends on the version of the .NET Framework installed on a machine. What do you use image comparison for? Probably, using other ways to validate data (e.g. Object or Property checkpoints) will work better for you.
A related question is, what method does TestComplete use to capture screenshots?
You can use the SaveToFile method of the Picture object. Please find more information in the Picture.SaveToFile help topic.