ports

hi,

how do you open and close ports? do you need a software? or web-based config of the the hardware involved can be used?

I have a medium sized network ( peer-to-peer ) and i hear and read about this security things about ports being under attack.

most of the operating system are win 98 SE/ME, i have to Red Hat Linux 7.3. I have a router (linksys dsl router ) w/c claims it is also a firewall, it can be configured web-based. I have 3 24-port switches.

all pc uses norton anti-virus 2003.

im i under attack, how can i protect my network? or am i just paranoid.