I tried to imagine what you want to do but I could not fully understand.
Try this configuration:
Building1:
DLS-Line <---> DSL_Modem<--->Wireless_Router<--PC1-Wired and PC2-Wired
PC1 and PC2 get dynamic IP address 192.168.0.101 and 192.168.0.102 (assuming these IP addresses - could be 192.168.11.101 and 102 depent on router the leases IP addresses)
Building2:
The PC3 with Wireless card and Wired NIC (Windows XP - Don't enable briged)
and PC4 with Wired NIC
Set Wirelesss Card wirelessly associates with Wireless router (within the wireless coverage range) you will get dynamic IP address 192.168.0.103 (assuming)
Set Wireless card properties be shared to Wired NIC (or just be shared I don't remember exactly)
set PC3 Wired NIC Static IP 192.168.3.3, 255.255.255.0 gateway, 192.168.0.103
Connect PC4 wired NIC to PC3 wired NIC (if there is not hub/switch in between then you need crossover cable)
set PC4 wired Nic IP address 192.168.3.4, 255.255.255.0 gateway 192.168.3.3
In PC3 must be on in order for PC4 get to PC3 and internet or to PC 1 and PC2
PC1, PC2, and PC3, accessing/file_sharing between them can be by PC name (recommend a used of IP addresses). PC1 and PC2, can not access to PC 4.
YOU don't need the router in Building two for this configuration.
IF PC3 is Windows 98, try to install or use the sygate software. set wireless NIC and broadband (WAN). For better user, learn how to use sygate. it acts as gateway/router/shared_connection WAN to LAN or PC3 Wireless_NIC1-NIC2 to PC4 or PCx
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Hope this help. If the configuration is not what in your intend, then I would suggest you to draw the picture jpg and attach in the thread/post. Or at least use the method and naming PC#, router #, location. DON"T put everything in one or two paragraphs.