Wireless Network file downloading problem

Wireless computer (win xp pro, 320 MB ram, amd 500 mhz) with linksys wmp54g wireless card. Machine accesses linksys wrt54g wireless router configured with firewall enabled, mac address filtering, dhcp enabled. Have spoken to linksys on several occasions, upgraded router firmware twice, wireless driver once. I have very good to excellent signal strength. From the wireless computer I can not download files - or open email attachments - for example I tried to download adobe reader - it looks like it starts off ok and then when the file transfer is to occur it stalls and ultimately informs of a problem. Is this a problem with ftp through the firewall? Do other wireless routers experience a similar problem? Any info is greatly appreciated.