Try it without security at all. It takes less than 10 minutes to crack WEP with freely available software; consider updating your security to WPA2-PSK [AES], or WPA-PSK [TKIP] at the minimum.
Update the firmware on the WRT54G. Look at the barcode sticker on the bottom of the router for the version information - that's the only place it tells the router's version.
That can help connections with vista machines as well as updating security capabilities.