have you got a software firewall running if so you will need to configure it to allow network traffic
to add a printer the easiest way is to hook the printer up to a machine xp preferable once installed go into the printers panel right click the printer icon for your printer click properties select the sharing tab check shared as assign the printer a share name in the box to the right eg myprinter-1$.
the next stage is to map the printer to the other computers. To do this go into add printers on the client machine click on add printer, the wizard will ask if you want to add a local or network printer check network then next you will be asked what printer you want to connect to check connect to this printer or( browse for printer) leave name box empty click next, you will then get a box up showing machines on the network double click the name of the machine the printer is connected to this should then reveal the printer, click on the printer name then click next do you want this to be your default printer y/n click next then finish
finally check properties on both printers and check all paper settings are A4 as many default to letter