Can not transfer from iTunes to iPad

This has just happened. Before I have had no problems transferring my Apps purchase from ITunes to my iPad, but all of a sudden I can not. I can transfer to iTunes with out a problem, but not the other way round. Can sync OK when connected. I have had a few windows messages (not all the time) saying that iTunes has had some problems and needs to close down, but only when I have shut it down not when running. This also happened when I could transfer the apps. Any idea? Running iOS 4.3.3 have tried reinstalling ITunes, restoring from back up also reset warnings, but this has not fixed problem. I am using windows XP same problem on my iPod Touch

Thanks