Desination net unreachable

I have a lease line connected to Cisco 1841 router. I have conneted the Fastethernet0/0 to SMC Wireless Router's WAN port. I access the internet from a computer with Wifi Card installed. Sometimes when I try to ping any site I get the reply:
Pinging 61.1.128.5 with 32 bytes of data:

Reply from 192.168.2.1: Destination net unreachable.
Reply from 192.168.2.1: Destination net unreachable.
Reply from 192.168.2.1: Destination net unreachable.
Reply from 192.168.2.1: Destination net unreachable.

Ping statistics for 61.1.128.5:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 0ms, Average = 0ms

Where 192.168.2.1 is the default gatway of the PC. I have used DHCP server of the Wireless router for internal LAN.

It works fine sometime.
Pl. suggest me the solution.

hi
have you tried logging into the router and checking whether you can ping those sites from there?
also let me know if any other devices i.e computers connected to that router is working ok or not.
your description of the network structure is not very clear to me. if you could also show how exactly the cisco 1841 is connected to your internal LAN, it would be of great help
cheers

Yes I can PING the sites from Router. My internal LAN is connected through a Wireless router. I am using WAN port of Wireless router to connect to FastEthernet0/0 of Cisco 1841. All the Computers are with WIN XP Home edition with SP2.

Just to clarify,

When this problem happens (or is happening), you can ping the 61.1.128.5 address from the 192.168.2.1 router ?

yes i can

How does this router get to that IP Address ? i.e. is that the router that has the connection to the Internet ?

Do you have any routing protocols running ?

As Arani said, could do with a little more detail on the network architecture.

Cheers