PC automatically goto out fo network

There is no reason for your computer to disconnect from the LAN, unless you access it through a remote (dial-up) connection.

If you are directly connected to the LAN, I would suspect either a physical defect on a NIC, or a problem with you Windows 98 configuration.

If you are connected through a dial-up networking, the disconnection is almost unavoidable. You would need to manually re-dial the connection.

Your explanation about pinging is incomplet, the ping command will always work regardless of the connection status, only the actual answer will differ. "reply properly" is insufficient.
Does name resolution work?
You can ping either an URL or an IP.
If you ping an URL and it returns an IP, then name resolution is working, meaning that your PC is still on the network, and therefore has some kind of local system problem.
If it does not return an IP, then you are not on the network, and look for a possible hardware problem, first.

Hi

As Christian says above, the problem probably lies with the PC's configuration. Check the points Christian has made.

Next check that DNS is setup correctly. Look in your TCP/IP settings - the first DNS server address should be the address of the domain controller where the DNS server is set up. The second DNS server should be a fail-over DNS server or your gateway (internet) address.

Next, make sure that your Win98 machine has a valid computer account. If you are not sure whether this is the case, delete the existing account and create a new one.

Lastly, make sure that the Win98 computer is healthy. Check the OS is fine by running SFC and make sure it has plenty of disk space.

Let us know how you get on

Good luck