for every once in three days i'm facing this problem. in the tool bar a icon appears saying "new hardware found" & i have already installed realtek... but when that icon appears in device manager there will be a sign '?' before ethernet controller, to overcome this i have to restore my system this is very frustrating since i have to restore in once in three days... how can i solve this problem please help me

you are not realy giving enough info
try 2nd button on realtek in device mamager and say update driver , point the updater to the possition where your realtek file are.
post a screenshot of your device manager with full details

you didnt mention whether this affects network connectivity. what happens alot of times is registry gets corrupted

go to system in control panel, hardware then device manager. click the + sign to the left of network devices , you should see your network card, right click click uninstall, either go up highlight the name of your coputer in the above list click action then rescan or restart your pc.