A difficult choice indeed - newer with poorer specs or older with better specs ! If you opt for your dad's one I would certainly do something about the OS since there has been no support for XP for the last two years. If you are merely looking for a learning experience you might want to consider giving Linux a try. I finished up putting Mint 17.3 on to the notebook I mentioned above. I had a topic about it running in the Linux section, you might find it useful to have a look at this -
If you opt for the computer with Vista and still believe there is a malware problem with it then you should start a topic in the section 'Am I infected ?', but read the guides at the head of the section first - this is not the correct section for dealing with malware problems.
Your cable connection problem. There are three possible areas of cause: The cable or socket may be faulty; the drivers may need to be re-installed - go to the mobo maker's web-site for these; or the chips responsible for ethernet may have failed - this would mean a mobo replacement. Again, the best thing for you to do is to start a topic in the 'Internal hardware' section of BC on this. This section - the Vista section - is meant primarily for problems with Vista itself.
I wish you the best of luck with this project, do post back and let us know what your decision is.