some if not most of these solutions have been posted, but forgive me because ill list the steps:
1) have to be sure the computers are on the same network, if not change it and restart computers.
2) ensure the folder(s) or drive(s) have the permissions set. i usually put everyone can read, but not write so that way i dont have to set permissions individually. besides i check my logs on the router everyday to be sure only computers i recongize are issued an ip.
3) turn off windows firewall because its useless. if you need one i recommend using comodo or zonealarm, which both are free and infinitely better than windows.
4) allow file sharing on firewall and open ports if required. if you dont know, search on google or something to see which ports are used. dont remember off hand.
5) enable file and printer sharing if on vista.
for printers. install new printer on the computer you want to access via ethernet. choose network printer. browse for it and it should pop up. if not try install basic drivers for the printer first. this helped for an hp printer i had issues with.