Problems with threads and windows

[SkripT]

Hi all, I'm using Qt4 opensource on windows XP. This time I have a serious problem with threads and Windows: the aplicattion consists in a wizard of three pages. The first page the user chooses some parameters. In the second page, I start a thread that makes some process on the images that the user has selected in the previous page. That process consists in rotations and scaleds of the image. I start a thread that process the images in background. When the thread starts or finishes the process of every image it emits a signal and I connect this signal to a slot of the main thread in which I show a message with the result of the process in a QTextEdit Window, and the progress with a progress bar. Well, all goes perfect if I let the pc alone running only this program. The problem comes when, if I minimize the program and I try to do something else, the wizard crashes with a message related to Qtgui4.dll. Moreover, if when the thread is running (is processing the images) I try to scroll the bars of the textedit window, the program crashes too with the same error message Very strange. The next page of the wizard I make a thumbnailing of the images to see the results: I start a new thread to resize the images and I emit a signal with the thumbnailed image. At the main thread, I connect this signal to a slot that simply fills a QListWidget with the images that receives. Well, my surpise comes when, if I try to move the scrolbars of the QListWidget when the thread is running, the application crashes like before
I really will be very grateful if someone has the same problem and knows how to solve it or If anyone could tell me if I'm doing something wrong. I think that the code is ok because, as I have said before, if I let the machine running only the apliccation, all goes ok.

thanks.