I have a computer with linux wich i do the server with and i have another one with XP i need to block the computer with XP cause people just come and use the internet making the whole thing crazy but i need internet on that machine is there something i can set on iptables so that it can accept or refuse an ip address whever i want it to?

Do you mean the XP connects to the internet via the linux and there are times you want the XP still to have access to the internet but nobody using it or there are times you just don't want it to have access to the internet?

If it's the second one my brilliant solution would be to either

pull the network cable out of the card that's connected to XP (if that's how it's set up)

or do

ifconfig eth0 down (or whatever interface it is)

then

/etc/init.d/network restart

when I wanted it going again

If it's the first scenario not too sure

You could have two firewall scripts. They would be exactly the same, except that the one that would block the XP machine would have something like

iptables-A INPUT -i ethX -s 1.2.3.4 -j DROP

Where ethX is whatever card the XP comes in on and 1.2.3.4 is the ip address for the XP machine.

Whenever you want to block the XP machine, you just run the script with this line and when you want it to have access you run the script without that line.