I have the exact same problem, the only difference is that the wireless connection has worked previously.

The setup: ZxXel modem / router -> Linksys wireless access point. 64bit WEP encryption.
PC 1 -Static IP address, Subnet and default gateway. Belkin wireless USB adaptor. XP home
My laptop (pc2) -Static IP address, Subnet and default gateway, Netgear wireless USB adaptor, xp pro. connects first try to the ap, with static IP.
I try installing PC1 with my known working netgear adaptor, same result.

The problem PC will connect to the access point, but I cannot ping the PCs on the LAN or the gateway. I can however ping PC1 from my laptop (pc2). I can also get a reply from PC1 with net view \\192.168.29.102 issued from my laptop.

PC1 will connect to the LAN / internet correctly via wired connection.
I have tried uninstalling and reinstalling various windows networking components. I have also repaired the tcpip stack with netsh int ip reset (logfile) and repair winsock2 with the netsh winsock reset.
I have tried the lspfix program, but that reported no errors.

There are no virus / malware on the system...

I know the PC is talking to other PCs on the LAN, but something in the O/S is blocking outbound communications, and its doing my fruit in!!

Fuzz