MikroTik

I am using Mikrotik 2.9 version as my firewall and dhcp server.
just 2 days ago i notice 2 out of my workstation cannot browse again. One with win Xp and the other with Vista
They both acquire ip address from the mikrotik. When ever i do a repair on The LAN in Xp it repair and acquire ip Address but cant browse
In Vista after diagonising, with will complain that it cant communicate with the DNS sever. I try to connect to other network and through a linksys router they all browse. But through the mikrotik server i cant browse. No firewall no restriction was created. Pls help

Did you enable remote requests in the DNS server on the RouterOS?

Can you ping the RouterOS machine? Can you ping some other external IP address? (not DNS name) like 206.190.60.37 (yahoo)

I dont understand what you mean by remote request. I can not ping external ip address. Even the DNS i cant ping it.
I can only ping the local subnet i.e the local workstation.