I loose my internet conection

I dont talk very well english but i hope you can answer my question.
In first i already saw a a question similar to this but it wasn t no answer.
My problem is that i enter in xp and have internet but sometimes after 1 hour, 2 hour … like the day to nigth i have no internet but the worst is that the problme is not from my isp not even router because i just reboot my pc and sart to work fine again. I think that is from windows xp but i cant format .
please try to find a way to fix this.