I am having a similar problem to mburns_df, I recently formatted my computer. After windows had installed I proceded to download all my drivers and all the windows updates (Im running XP Pro). After doing all this I found that I was getting a connection to the internet (I can ping sites etc) but internet explorer wouldnt work, msn messenger wouldnt connect and anything that required a connection wouldnt come on. I rang BT and they went through some basic tests but when they had verified that my connection wasnt at fault they didnt want to know. They did recommend that I uninstalled service pack 2 though, I tried this and then everything was fine, everything connected. After about an hour everything started to go again, IE became erratic and sometime opened sites and sometimes didnt, programs like msn messenger would work but only if they were run before it all went dodgy. All this became gradually worse till nothing would work again. I checked the forums for help and found a program called Winsock Fix, I ran this to reset my tcp/ip and to reset all the registry entries effectively resetting my entire connection. This worked perfectly but again, after about an hour or so it all goes erratic again. I really dont want to have to format over this but I dont know what else to do. Has anyone had any joy from this problem?

By the way, no firewalls are running only my antivirus (norton 2003)

I would suggest that you try doing an extended ping with loaded packets to your ISP's default gateway and then to a known responder. You may have a connection problem despite what the techs said. Do it like this from a command prompt:
ping daniweb.com -l 1400 -n 100

the -l is a lowercase L this command will send a ping with packets loaded with 1400 bytes 100 times. You should get no errors going to your default gateway. If you get any dropped packets then when you actually try to run and application it may fail even though a default ping answers. This happens a lot by the way.