I have configured Sendmail/Fetchmail and Samba Services on same Redhat Linux 7.1 server. When dial-up connection is up and fetchmail is running MS Windows Clients are facing connectivity problems, as soon as we disconnect dial-up link clients can connect immediately.

Can you ping the linux box from the windows boxes while dial-up is going? When you start up your dial up networking your default route becomes the gateway your ISP has given you from dhcp. This really shouldn't be a problem if your nic card has an ip in the same subnet (subnet mask matches windows subnet mask and the network portion of the ips match) as your windows boxes. The linux box should know explicidly where the route to your network is because it has a nic in that subnet. The dial-up does become a problem if your machine gets to your local network just by broadcasting (because it doesn't know the route to your network) most of the time and then when you start up your DU it has a default route so it tries to send everything out it. Possibly putting in a route/routes to your local network/networks may solve your problem. You can do a netstat -r to see your current routes then do it again when your DU is running. Do a traceroute from the linux box when your DU is up.