..................agh...... that screwed up everything and I had to reset my router... gaaaaaah...... and anyway I don't think that could solve the problem, since the problem is not directly file and printer sharing, the computer just cannot be pinged, therefore file sharing doesn't work

And the computer is getting the correct IP from the router, so I don't think that setting the IP's manually would change much.

Also, I don't think the problem is because of the router ; since it worked fine between a 3rd computer on the network (with XP Family) and comp#2 (also with XP Family) ; I am 110% sure that if comp#1 would have XP Family it would work right away... but then again, it has XP Pro and there has to be a way to make it work