Howdy merlin
When you start experiencing this "lag" try the following, go to the dos prompt so Start -- RUN -- CMD and press enter
Type Netstat and then press enter.
This should show a list of everything running, if you are browsing sites they should show up + if anyone is connecting to your machine you should be able to pick up their address.
You are obviously up to SP2?
To resolve the problem you could try some of the following (you might have already done these but I will list them):
- Go into your Windows Firewall and reset everything to default
- Run a registry cleaner just incase something has been added to the registry that would allow access to your machine
- You could try to uninstall, then reinstall your network card / wireless network (so that everything is reset to default)