I have here a windows on a full encrypted hd. I cannot disable this encrypted hd. Everything should be unchanged, I just want to Install Ubuntu on an extra partition. But here I think the problem is: if the full hd is encrypted I cannot borrow some space for an extra partition right? So what should I do? :/
Your best bet is to install Ubuntu using Wubi. It's not ideal but it's the only option you have (that doesn't include losing your Windows install).
The only other option is to get a new hard drive or to completely wipe Windows.