It is very less probable for a problem caused by software problems. When such things happen, ppl first think that it is because of a virus, host addresses, operating system problems, etc...
Its more likely that an hardware problem such as phone line joint disconnection, a bill hasn't payed to Internet Service Provider,... etc...
First try to install another web browser program such as Firefox, Opera, ... whatever and try to surf by using this program. If problem persists, than I can say that
Problem caused by a virus, operating system IRQ's , hosts IP's..etc : %15
Problem caused by ISP or their bills : % 20
Hardware problems and dis-joined line cables: %45
The other problems that I don't know, known by geeks: %20
