how can i block a particular or group of sites in a pc

You can block them in the firewall in your router, read the router manual on how to do it...

what if i am using dialup

Build a hosts file and insert it in as a replacement...The problem with Hosts Files is that they can become so large so as to slow your system down.

What is the Hosts file?

You can block websites in software firewalls as well.

I even use advert blocking software to deny access to websites...

yes and there are other programs that can be used. The poster needs to explain as to what type of sites and how many they need to block so as to determine the best tool for the job!

All you need to is insert something like this to block Facebook, or any of the other crap social sites in a advert blocker *.facebook.com/* and set the filter to block retrieval of url.

In a software firewall it is a little more involved, and requires you to read the manual on how to do it correctly...

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Wombat's: Bulldozers of the bush...

We would need to know what type and why.
If it's just simply to block certain content or specific urls it can be much easier with browser controls.

How to Use the Parental Controls on Internet Explorer | eHow.com

Parental controls