Is it only when using the internet or using other products as well?
Solving problems about a PC freezing is not easy as it could be a load of different things. Do you ever get any messages before or after this happens?
It can be caused by you not having enough memory and Windows will grind to a halt while trying to recover from this. Does the hard disk light flash a lot when it is happening?
Do you know how much memory you have, and have you got plenty of space on your hard disk?
It may be the computer getting too hot. Has it got enough space round it to let in air and keep it cool. Could it be clogged up with dust or fluff. May be a good idea to take it outside and blow the dust and fluff out with a hair drier (set on cool).
It may be you have a bad driver for one of your hardware components (graphics, sound etc). Have you installed new hardware lately, or updated any of your drivers?
If it is none of those things get back to us and I will see if we can suggest anything else.