WRT54G Wireless Router Problem. Need Help!

Question

I'll start with the basics of my situation and if more details are needed, I will provide to the best of my ability.
  I have a comcast cable internet connection.  I have 6 computers in my house. Two of them are downstairs (which is where our modem is.) The other four computers are upstairs.  All of these computers are Desktop computers with the exception of one laptop.
My laptop (which is Windows XP) can pick up a random wireless signal from the room it is in.  I also found out a way to share the internet connection on the computer by going into the properties of my network connection and enabling internet sharing on it.  This means if I connect a network cable from my laptop to another computer, I can leech the connection from my laptop and have internet access on the other computer.  I also  had a switch that I used to connect the other 3 computers upstairs by means of the laptop.  The problem was, the laptop was using someone else's wireless connection in the area, and it wasn't strong enough to keep up a constant connection.
I decided, if I can leech someone else's wireless connection, why not have my own.  So i recently bought WRT54G Wireless Router and set it up on one of the computers downstairs (because that's where the modem is).  It works fine as far as connecting my laptop to the internet from upstairs.  My laptop can access the internet through the connection I set up in the LELA program. 
Here is the part I am confused about.  Everything is plugged in correctly as far as the router is concerned because the laptop can access the internet.  But when I connect an ethernet cable directly to one of my other computers (which are Windows Vista, but I didn' t have a problem with it previously when I was leeching someone else's connection), the connection drops on my laptop, and the other computer still can't access the internet.  A notice pops up that says its trying to identify the network, but it gives an error that says there is something wrong with the DNS.  As soon as I disconnect the ethernet cable connecting my laptop to one of the other computers, the connection is fine again on the laptop.  It's only when I connect the computers together that the connection drops. 
I just don't understand why it wouldn't work when I created my own network if it worked with another network with no problem.  My whole point in getting a wireless router was to have a router within good range to keep up a constant connection on my other computers that aren't in range of the modem downstairs.  I just don't know where else to look.  I'm not sure if it can be fixed on LELA, manually through my wireless settings, or if it could be a problem with one of my computers' settings.  I've tried many different things and I'm not sure what else to do so any help is most definately appreciated.  I feel like I've tried everything.  I know it's possible because I've one it before, I just need the help because I'm not much of a networking person and I don't know what I should start with to make this work.
Thank you to anyone who can help or even point me in the right direction.

Answer 1

Access router setup page ...Click on the Wireless tab on the Setup page- Here Wireless Network mode should be mixed- Provide any non-linksys name .... Set wireless channel to 11- And wireless SSID broadcast should be Enabled and then click on "Save Settings" >>Now Click on the Sub tab under wireless > "Wireless Security" Change the Wireless security mode to WEP/WPA ...type in the key & click save settings .... >>>>Click on Advanced Wireless settings>>Change the Beacon Interval to 75 >>Change the Fragmentation Threshold to 2304 Change the RTS Threshold to 2304 >>Click on "Save Settings" Check the connection once again & see if it works or not ...