problem with subnet mask

[klenshin]

hello i have a PC (with windows xp pro sp2).i use internet by a DSL modem and i connected modem to PC by a network cable and when i turn on modem the PC connect to DSL ISP by modem and  get IP automatically from ISP and for example:
ip:1.202.100.120 subnet mask is:255.255.255.0 and gatway is:1.202.100.1 and i use dynamic ip for internet like:86.120.65.84
but i installed ESET smart security and it's firewall changed my networking configuration and i lost my internet connection and ISP connection then i uninstalled ESET smart security but my previous network configuration did not comeback.
now i turn on the modem for connect to ISP ,it can not get a IP from ISP for me nad i receive this error( Limited or no connectivity) when i check details of this network
i see in this my IP is:169,254,120,100 and my subnet mask is 255.255.0.0 and my gateway is my IP .i know it that my subnet mask must be 255.255.255.0 but this is not i can not configure TCP/IP setting manually because it must be auto from my ISP
how i can fix this problem .how i can reset all IP setting that stored in windows how i can set 255.255.255.0 for my connection subnet that in another time dont change to 255.255.0.0 
help me please
thank you very much
 

[GX44]

Not entirely sure this will work, but try this:

-Click on the start menu and  click "Run"
-Type cmd and hit enter
-type ipconfig /release
-power modem off for a minute or so
-in the cmd box, type ipconfig /renew

Hopefully this will obtain an IP address from your ISP and get you connected again.

[klenshin]

thank you
but it do not work i used it before even this command(netsh int ip reset reset.txt) too
and they do not  work
my problem is here how i can reset or clear all TCP/IP configuration on windows perfectly !
 

[viking]

you may reinstall network card drivers.
Why do you stop/start the modem?
Are you sure that ESET smart security caused all the trouble?

If you received IP automatically and now you don't, you don't have to mangle with IP settings. If its IP is 169.254.x.x (by the way, for this IP the subnet mask is fine, 255.255.0.0) than the computer is not able to communicate with the DHCP server. Maybe it is a problem with your network cable. If you checked the network cable, you may try with a laptop/another computer to see if works fine (if your ISP has a MAC filtering policy, my last idea will not work).
To see if it is Windows/network card drivers to blame, you may check with an alternative operating system like Ubuntu (http://www.ubuntu.com/getubuntu/download) or Knoppix (http://www.knopper.net/knoppix-mirrors/index-en.html).