My iTunes won't work with my Windows Vista!?

Ok, so I installed a iTunes for Windows Vista a few days ago. It worked fine until yesterday. Yes my iPod touch is perfectly fine and iTunes pops up when I connect it to the computer but whenever I try to turn on iTunes the computer stops and it and then it says something called DEP stopped it! I tried to turn it off but the computer said iTunes could only work with DEP turned on. HELP!!!!