i have a windows 7 and it wont complete the sync with my iphone4.

My device will not sync with my iphone4, rather it does not complete the sync, is sends a message saying that "itunes could not sync to iphone because it has been disconnected".. i have tried removing vidoes to see if maybe one of the features are preventing the sync function but now that i have it just stops responding before it actually starts the sync, any sugestions?