<p>I'm building Desktop Software for over 10 years now, mostly it's simple Data-Input Software. My problem is, it's always looking the same: A Treeview on the Left and a lot of Text/Data Fields to the right, depending on the type of data currently is worked on. Are there any fresh ideas how such software nowadays should look like?</p>

<p>For further clarification:
It's very hierarchical data, mostly for electronic devices. There are elements of data which provide static settings for the device and there are parts which describe some sort of 'Program' for the device. There are a lot (more than 30) of different input masks. Of course i use combo boxes and Up/Down Entry Fields.</p>

Having all of your software look the same thing is a
good
thing.  One of the best ways to make it easy for people to use your software is to make it look exactly the same as other software your users already know how to use.
There are basically two common strategies for how to handle entry of a lot of data.  The first is to have lots of data entry fields on one page.  The next is to have only a few data entry fields but a lot of pages in a sort of wizard-style interface.  Expert users find the latter much slower to use, as do users who are entering data over and over again.  However, the wizard style interface is less confusing for newer users since it offers fewer elements at once and tends to provide more detail on them.
I do suggest replacing as many text fields as possible with auto-complete-based combo-boxes.  This allows users to enter data exactly the same as with text-boxes, but also allows users to save typing by hitting the down key to scroll through choices after typing part of the data in.
Providing more detail on what data is being entered would probably yield more specific answers.