DHCP-LAN OK, but no WAN, Static IP - LAN & WAN OK??

Hello,
I have an ethernet based network at home - multiple PCs (XP-SP2 & Vista Ultimate based) - CAT5E cabling throughout - DHCP served by linksys BEFWS114 - using a Dell Powerconnect Unmanaged switch for different rooms. Wireless is disabled on the BEFWS114.
The setup has been working fine (all devices on same subnet) - n/w printer & NAS had static IPs while the PCs had dynamic IP (DHCP enabled) - everything was working fine for over a couple of years without any problems. Out of the blue, last week, the PCs just wouldn't connect to the internet, but could access the other devices on the LAN ok. Tried to restart the PC, release and renew the IP, no problems there, but the PC just wouldn't access the internet. I forced it to a static IP and it worked fine! I have no idea what caused this - this is the same case with ALL pcs - two laptops (Dell Inspiron/Latitude), one Dell desktop, two custom built PCs (ASUS P5WDH-E6600-4GB, MSI-NEO2PLS-P4Prescott2.8GHz-2GB), so I don't think it is related to a PC setting.
I then tried to disable the DHCP on the linksys and then connect a Netgear WGT624 to the Dell switch and enabled DHCP on it - same results.
Any idea as to what the problem might be?
Thanks