I should like to connect two Windows XP computers
via Gigabit ethernet ports. What a software is necessary
To ensure the possibility to use one of the PCs as a terminal of another
With sharing HDDs ets?

You'll just need a crossover cable (e.g. http://www.maplin.co.uk/Module.aspx?ModuleNo=45887&doy=9m11D). You won't need any extra software. Windows should be able to deal with that.

As for sharing hard drives etc. that's also very easy. This should get you started: http://www.practicallynetworked.com/sharing/xp/filesharing.htm