problem with MTNL wireless modem 450TC1


Hi all I am facing a grave problem with my MTNL modem.I am using MTNL wireless modem 450TC1. I am having windows XP on my desktop and laptop.Problem is whenever i connect my modem with my laptop using a LAN cable it detects the setting (TCP/iP) and connects to server and function well.TCp seeting are : obtain an IP address automatically.MY ips are DHCP enabled. But problem arises when i connect the lan cable to my desktop. When i activate the LAN connection it tries to find the IP address but unable to find any and connect with deafult 169.24.XXX.XXXand it shows Limited or No connectivity. I am unable to open any site. when i try to open https : .... it does not get open and i get an error internet not connected.When i ping the i get Request Time out.I try giving the manuall Ips But in that case I am not getting " Limited or No connectivity " error but still I am not able to open any site not even https : i ping to the server i get Request time out error. Story does not end here ... At times ( very rare) My desktop gets able to find the correct IP address ( TCp seeting are : obtain an IP address automatically) and it got connectd ans starts working .. strange !!!! It will work for that full day but as soon as i shut down and start PC again and ty to connect to net I will not work ...I seriously don't know what to do .. Could someone please help me out ...Is there anything wong with the settings of desktop ????I forgot to mention .. My firewall settings in my desktop are off ... Thanks