You say you have Broadband but don't say if this is cable or ADSL. Also are you trying to network using cables or wireless (or a combination)?.
If you already have a modem, you only need a router,
but may prefer to get get your own combined one.
Not all routers support printer sharing so you need to check this out.
If you want to go wireless route it's a lot easier to set up if you have XP on all machines as you can clone settings from one on USB drive and automatically transfer onto other machines.
Lastly, Wireless Routers normally come with some sort of encryption security but the level varies amongst manufacturers ; some have inbuilt firewalls.
I suggest you research forums first before shelling out hard earned readies but around £200 i'sh outlay should cover it.
Try click here
it should answer some of your questions