Currently I am having problem with my openvpn ( Linux based ) setup where my windows client not able to connect to servers which are behind openvpn server.

I am able to ping both the interface i.e eth0 and tap0 but unable to ping any machine behind the openvpn server.

OpenVpn server route is like this

Kernel IP routing table
Destination Gateway Genmask Flags Metric Ref Use Iface
192.168.1.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0
172.16.10.0 0.0.0.0 255.255.255.0 U 0 0 0 tap0
169.254.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0
127.0.0.0 0.0.0.0 255.0.0.0 U 0 0 0 lo
0.0.0.0 192.168.1.1 0.0.0.0 UG 0 0 0 eth0

Client getting the default gateway which is IP address of tap0.

The first thing I see is that you are using a tap interface. That's a low level ehternet emulation device and usually you use it to create bridged connections (br0, bridge-utils.... Link aggregation).

Reconfigure your OpenVPN server and client to use a tun interface and try again. We can start from there :-)