Connection ? to Wireless Network with W98 Notebook

Recently set up a wireless network at home. Set up includes cable to cable modem to Linksys Wireless Router (WRK54G). No computers are hardwired to the router (except when I configured the router w/ my Windows 98 laptop). Have a desktop (Windows XP) and two laptops (WIndows XP and Windows 98) connected wirelessly to the router. Only the XP machines can connect to the network/router. The Windows 98 machine cannot connect. For that computer I use a Linksys WPC54G adapter. It did work fine at one point. I later performed many overdue Windows updates. I also added MAC filtering and WEP encryption. After doing these two exercises (updates and security) the Win98 machine won't connect. I've double checked the MAC addresses in the filtering. I'm sure the setup (encryption key) is correct (it's the same as for the two machines that will connect). The laptop surveys and finds the network (signal strength good 90% +). I also uninstalled and reinstalled the Linksys driver/software after doing the Windows98 updates and that didn't help either.

Any ideas on what could be the problem here?

Tony