Really easy, install windows xp with sp2, once setup, turn off windows updates ie notify me but dont dont automatically download and install, you may be receiving a hardware driver update which is conflicting with your LAN hardware. Easy fix dont automatically download.

If the problem was IE related or firewall or span, you can test this by going to start and then select run, type in cmd and Open it to run. You will receive a black command window, type ping www.google.co.uk and press return. if you get a response your internet is OK. Then type ping 209.85.229.104 if you get a response all is well. Now load you IE and in the address by type 209.85.229.104 and press enter, if google comes up all is well, if so now try www.google.co.uk if it doesnt load, you know you are having issues with resolving domain names to ips.

Though unrecognised lan connects, is down to driver issues, especially when it did work and then suddenly not, a conflict in the hardware driver, not a faulty ethernet cable etc. If a reload from scratch resolve then you know for certain there is no hardware issue, kill the updates and all will be fine, microsoft drivers updates verses Dells drivers dont always agree.

If it happens again, go to control panel, and select device manager, from network adapters, select you LAN hardware, properties and role back the driver, not uninstall, but role back to previous driver.