Winxp Home Internet Connectivity Issue on Home Network Using Wi-Fi

I have 5 pcs networked at home. PC1 is XP, PC2 is ME, PC3 is 2k, PC4 is 2k and PC5 was recently upgraded to XP from 98 (and was not on the network until after the upgrade). I'm using Roadrunner with a Toshiba modem. I have a Linksys BEFW11S4 Wireless Router, and Linksys Wireless adapters in PCs 2, 3, 4 & 5. PC1 has a Linksys WMP11 PCI Wirelss card and a PCI Ethernet port.

From the wall, the cable enters the modem, the modem has Ethernet to the Router, Router connects via Ethernet to the appropriate port in PC1.

This architecture worked fine when we had only 3 PCs networked, our main goal being to share the internet connection. File and resource sharing were not a big priority.

Added PC3 (replacing an older, ME box) and PC 4 (new connection) very easily. Can access internet, can "see" other PCs and printers, shared folders, etc. PC5 however, has been a large headache.

First, no ping between PC1 & PC5 or vice versa. Troubleshot, added ICS for Wi-Fi card, and now could ping both ways. ((During troubleshooting, one configuration would allow PC5 to ping PC1, but not allow PC1 to ping PC5--very strange)). Next, discovered that I had shared the incorrect device, so I deleted the shared connection on the Wi-Fi and added sharing to the Ethernet card, since this is the device that will share its connection to the internet.

What do you know? It all worked!! Internet on all 5 pcs, name and ip pings all around!! I would have whooped for joy, except it was midnight and the wife was sleeping. Powered everything down, went to sleep.

This morning, I awakened and deemed it proper to display the "dragon" I had slain the prior evening. Powered everything up, and the wife was able to look over my shoulder to see the EXACT SAME ERROR screen from browser we had seen the day before.

Now I don't believe in internet fairies, or "magic smoke", or any other mystic properties associated with computers. But, somehow, overnight, my repairs had disappeared.

My first question: what the heck happened to reset my fix to this problem?
My next question: how do I re-work this configuration so that it funcitons?
My last question: is it possible to configure this network so that each PC will have an independent 'net connection without having PC1 on to enable the internet? If not, can this system be configured so that PC2, 3 & 4 can have independent connections and only PC5 will have to be dependent on PC1 being powered on?

I am not an IT professional, but have some rudimentary knowledge of how things work. I have visited Microsofts knowledge base, and about 150 different tech forums, as well as countless websites generated from my search engines to try and find a solution to this problem. All efforts have resulted in no solutions, lost sleep, and worn nerves.

While my ultimate goal is to allow each client its own, independent access to the internet, I will be happy if only PC5 has to have the host connected.

Please help.