I did as you said: I didn't apply any network tweaks, checked the DHCP compatibility options (i didn't use integrate drivers and hotfixes options). After windows reinstall, I installed only the drivers, Firefox and Total Commander. I've tested the connection but after 40 miutes the error occured again [with an original XP it works properly (tested!)]. So I don't think this error caused by any programs I use. So unforunately I cannot find the reason of my problem. I give it up (This error occured about 2 months ago and since I cannot fix it). Thanks everybody for help.