1) Yes just move the old drive in the new computer and copy the files over. Windows will see the drive, just open her up (in windows) find all of your files and copy them over. It can only be data files, pictures etc. Any programs will have to be reinstalled.
2)If Office XP has already been activated on the old machine then yes you'll have to buy a new copy or contact MS on what to do. If you plan to use Office XP on both machines, you'll have to buy a new copy. If its just on the one machine (the new one) phone MS tell them and they should help. (i think i'm right there)
If you have a laptop and PC only then you are allowed to use office on both with no problem.
3) Not to sure but use the rograms\Accessories\CommunicationsNetwork Setup Wizard to setup a network. (networking isn't my thing.