I am typing this from PCLOS box which is getting the connection from an XP machine which is connected to the net.
You will need to link up your linux box to the xp via a crossover cable and configure an IP address for linux using ifconfig.
Say you want linux to have IP adress of 192.168.0.10 then :
ifconfig eth0 192.168.0.10 netmask 255.255.255.0 up
You will also have to configure your linux so DNS and default gateway are 192.168.0.1.
XP - side ( need 2 network cards) :
When you ICS, your machine gets assigned the IP of 192.168.0.1, however, if your modem has already been assigned this then ICS will not work as you will get an error saying that 192.168.0.1 is already in use. If that is is the case you will have to network bridge the two network cards on you XP machine. When you ICS the modem will get an IP assigned by your ISP and your "network bridge" will get assigned IP of 192.168.0.1.
I wanted to do exactly as you and I posted my query on here. This is the thread.
You will find this more help ( it contains screenshots as well). Many thanks go to Mikasi for helping me out !