Not too difficult. You need the right hardware (either a hub/switch and two cat5 cables, or a single crossover cat5 cable). Each PC needs to have a network card in it, and you'll need the win98 CD to install networking stuff. Will already be on the drive for winXP.
Then the best step is to read up on how to do it. The site I like the most is
www.wown.com, although
www.practicallynetworked.com is fairly good as well.
Have a go at following the instructions on those sites, and if you get stuck, get back to us!
Cheers
Mick
Oh, and Welcome to TechIMO!