I would check with your ISP, as to whether there DNS servers have changed, or are having issues, as this would certainly show the problems your are experiencing.
If you check you IP configuration - run IPCONFIG /ALL from a command prompt, and see if you can PING the DNS Servers specified.
Examples below:
C
>ipconfig /all
Windows IP Configuration
Ethernet adapter Local Area Connection:
Connection-specific DNS Suffix . : medusa.mezzonet.net
Description . . . . . . . . . . . : Marvell Yukon 88E8055 PCI-E Gigabit
Ethernet Controller
Physical Address. . . . . . . . . : 00-17-42-1B-0B-50
Dhcp Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
IP Address. . . . . . . . . . . . : 192.168.44.124
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.44.1
DHCP Server . . . . . . . . . . . : 192.168.44.1
DNS Servers . . . . . . . . . . . : 81.171.143.18
85.133.19.2
212.227.39.54
Lease Obtained. . . . . . . . . . : 14 December 2006 11:45:21
Lease Expires . . . . . . . . . . : 13 January 2007 11:45:21
and
C
>ping 212.227.39.54
Pinging 212.227.39.54 with 32 bytes of data:
Reply from 212.227.39.54: bytes=32 time=20ms TTL=52
Reply from 212.227.39.54: bytes=32 time=19ms TTL=52
Reply from 212.227.39.54: bytes=32 time=20ms TTL=52
Reply from 212.227.39.54: bytes=32 time=19ms TTL=52
Ping statistics for 212.227.39.54:
Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 19ms, Maximum = 20ms, Average = 19ms