Hi gangchen,
We don't know what version you're using but maybe you should try a non-beta one. We don't know what browser you're using. We don't know a lot. We do know IDM has nothing to do with Windows 7 and this was posted in the wrong forum. However, I'd suggest you take up Mitchell's advice and find a program that works. Its possible YouTube changed their internal code to prevent people from leeching videos and your program couldn't compensate. Whether or not downloading videos off YouTube en-masse violates their terms is something you have to sort out. Its certainly not something we can support or even fix on our end.