I have a home network with 4 computers on it. 3 computers are hard wired to a router and the fourth (LAPTOP) is wireless. all use the same router.
COMPUTER A can not see any of the other computers. It can use a shared printer on another computer.

Computers B, C, and D all are working fine, and they see the shared files on Computer A, and the can use the printer connected to computer A.

Why is Computer A not seeing the other computer's files? OK we need more info:
What OS' are the computers?
Are you working with a workgroup?

Jonas I am using XP home and the workgroup name is the same.Sow you're all using XP home SP2?
What firewalls are you using?
MAKE sure network detection is enabled in the firewall settings.

Jonas How do you TRY to see the other computers?
Is a "NAMING service" on your LAN? If not, maybe you should create some records in the hosts file, on every computer (I will have to read a bit about Windows naming services, I can't help you more now).Uh, are B, C and D setup with shared files?