I have an interesting problem.
I am running a win server 2003 and win xp sp3 network. Until recently we have been able to access all network resources and the internet without any problems.
Now however, we can still access the network resources, (printers/ server files etc), the exchange email system and blackberry servers can send and receive messages, the antivirus solution is able to update trhoughout the network. The only trouble we have now is that we get intermittant web surfing, i.e. sometimes when we open IE8 and type in a page it will connect and allow us to browse that site (if somewhat slower than usual), others we get the can not connect .... even after refreshing the page.
I have checked the router and switches trafic using Wireshark and all appears to be fine.
Does anyone have any suggestions on how to proceed?

Download and run Malwarebytes free scanner. It will tell you if there is something redirecting your web activities.

Thanks rch1231 for the reply. I Ihave tried that ... all systems clean as a whistle... my first thought was virus / malware.
But as we have full access to all other internet related services (email through exchange, windows updates, AV updates etc), I'm thinking that it is likely to be a setting issue on the main server

Well I tried the quick and easy answer. Sorry. How about this:

Start with the basics and work your way up. Double check the cable is plugged in all of the way at both ends. You would be amazed at how many loose cables cause intermittent problems and look like they are properly connected.

What does ifconfig (ipconfig for windows) report?

Can you ping the computers network port?

Can you ping any other systems?

Does this just happen some time or all of the time?

Can you ping the gateway, and each of the nameservers?

Can you query the nameservers like this:

[root@cwsvr02 cur]# nslookup txlinux.com

Non-authoritative answer:
Name: txlinux.com

Also try traceroute (or tracert for windows) to an IP and domain and see if there are any bottlenecks along the way?

Post what you find and we will do what we can.

Thanks rch1231.

I can ping servers / workstations / printers etc all is working in that manner.

Now that i'm back in work, with out any other users the internet it seems fine. I've turned a few machines on connected throught to the internet without issues.

The issue may have corrected itself....
I will know better on monday when all of the users are back in.... if there are issues I will disconnect the machines and reconnect them one by one. This may give me a better picture of what is going on.