It sounds like a proxy problem? What web-browser are you using?
Can you still ping other websites?
Goto Start / Run and type cmd.exe, then type ping google.com or another website which isn't working.
If IE - Go to Tools | Internet Options and click on the Connection tab. Make sure there are no dial-up or virtual private networks that aren't spos to be there.
Also manage Add-ons and check there, nothing is been added which might be out of place.
If it cannot display on all web-browsers and is mostly targeting certain Anti-virus websites and Microsoft Update, etc, also attempts to disables your anti-virus scanner, but will let you still access paypal, bank websites, etc (mostly paid website)... then it's a virus, a newest one (which attempts to leech login details and bank accounts).
Possible 'Win32/Conficker' infected your computer? There are some varies of the virus.
It attempts to block everything that may help you remove it.
Remove the computer from the network and via another 'clean' computer transfer files (one way - once only) removal tools you can find on anti-virus websites.
The problem is you may not even be able to see this, if it has infected...QUOTE
System Changes
The following system changes may indicate the presence of this malware:
The following services are disabled or fail to run:
Windows Update Service
Background Intelligent Transfer Service
Windows Defender
Windows Error Reporting Services
Some accounts may be locked out due to the following registry modification, which may flood the network with connections:
HKLM\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters
"TcpNumConnections" = "0x00FFFFFE"
Users may not be able to connect to websites or online services that contain the following strings:
virus
spyware
malware
rootkit
defender
microsoft
symantec
norton
mcafee
trendmicro
sophos
panda
etrust
networkassociates
computerassociates
f-secure
kaspersky
jotti
f-prot
nod32
eset
grisoft
drweb
centralcommand
ahnlab
esafe
avast
avira
quickheal
comodo
clamav
ewido
fortinet
gdata
hacksoft
hauri
ikarus
k7computing
norman
pctools
prevx
rising
securecomputing
sunbelt
emsisoft
arcabit
cpsecure
spamhaus
castlecops
threatexpert
wilderssecurity
windowsupdate