Click on START
Click Run
Type cmd
Command prompt should come out... Type:
ping sg.yahoo.com -t
make sure that you're connected to the internet
IF The TIME TAKEN TO SENT 35 BYTES is more then 100 MS ( Milliseconds ), something is wrong with ur internet

My Case:

I got 2200 Milliseconds.....
Install Spybot and scan it and remove all infected datas.
restart com and all went well~~~

okay.
err, my internet, its perfect >.>

Care to do another set of instructions for Windows Vista?

Erm , i do not have Windows Vista computer, but i think the keys that is supposed to type in should be the same as its all Batch...