Whenever I plug in my iPhone 4S into my computer, I can't back it up. Help please.

It stays stuck on step 2 of 6 (sometimes step 2 of 5) when I try to back it up. I can't do anything with my iPhone, I can't add music or anything. I can't synchronize, nothing at all. I even kept it backing up for 8 hours, and it stayed stuck at step 2 of 5 when I came back - after 8 hours! Help me please!