only a tiny operation screen opens when I start Itunes in Windows Vista

In Windows Vista I am unable to open iTunes (updated 10.7); all that comes up is a small window in top left hand corner which enables me to press start. There is nothing else. It used to work well a few months ago but I downloaded a whole series, it worked badly with jittering sound and out of sync but after reinstalling both programs (iTunes and Quicktime) nothing is starting. Can anyone help, please?