Looks like my ISP has a MAC address filter on their end, hence newly installed wifi router was being blocked until I did a MAC address clone from PC to router using in-built function in the router's firmware: What is happening now is all devices connected wirelessly have access to internet, but the WinXP PC, connected via cable will not open any URL in any browser, albeit Skype works fine (I was able to make a call). I guess the question here is whether it is possible to get a PC working with router who's MAC address is the same? I would like to make it work without changing PC's MAC address if possible. This is because I would like the PC to be able to connect to Internet without the router when needed.
No. The mac address is used when deciding where network packets should go. You'll need to altered the client pc mac, or better, ask your isp to allow your router's original mac.