Some websites are not opened at all

Hi,
First of all:If there were a problem with my Internet network, then all websites wouldn't be opened.
I have been facing the probelm with all browsers I have been using 'Firefox, MS IE, and Google Chrome'
Last-night I have been opening any websites without any problems. However, today afternoon, I began to face the problem. So, if it were a virus, then I would be facing this problem last-night as well. I have been using MlawareBytes AntiMalware, and MS security Essential
However, I have noticed some websites are not opened, such as 'hotmail.com', 'Yahoo.com', https://www.adobe.com/go/flash-player-updatesMSN.com - Hotmail, Outlook, Skype, Bing, Latest News, Photos & Videos.

When trying to type those websites in address bar, then I encounter 'This page cannot be displayed'
Sometimes 'This webpage is not available ERR_CONNECTION_TIMED_OUT'

Also, some other programs installed on my PC, and when those programs need to be updated, then they prompt me to update them. So, when I chose update them, there is anwhich needs their

However, all google products, such as google.com, gmail.com, youtube.com, and so on are opened well.
Also, others such as sevenforums.com, and so on are opened well.

I have ADSL with a speed of 512Kbps
I have tried checking my cables and rebooting my routers, modems.
Could you please let me know why this happens.

Could anyone please take some of his precious time out to really read my posts and address this problem? I am quite anxious to know what might be causing the problem. Last-night I was opening any website without any problems. However, I only today have faced this problem. It still persists. This is strange. I am not posting you in this forum from my same internet network, and the same computer. However, other websites, such as Microsoft websites, Hotmail, Skype, and so on are not opened at all. Also, some others, such as adobe.com, other forums.

Sometimes this error shows me while opening some websites.

This webpage is not available

ERR_CONNECTION_TIMED_OUT
ReloadHide details
Google Chrome could not load the webpage because ar.wikipedia.org took too long to respond. The website may be down, or you may be experiencing issues with your Internet connection.
Check your Internet connection
Check any cables and reboot any routers, modems, or other network devices you may be using.
Allow Chrome to access the network in your firewall or antivirus settings.
If it is already listed as a program allowed to access the network, try removing it from the list and adding it again.
If you use a proxy server...
Check your proxy settings or contact your network administrator to make sure the proxy server is working. If you don't believe you should be using a proxy server: Go to the Chrome menu > Settings > Show advanced settings... > Change proxy settings... > LAN Settings and deselect "Use a proxy server for your LAN".

I am not using any proxy at all.

GokAy said:

An incorrect DNS setting will cause issues, yes.

I haven't checked that guide you linked, we have one of our own :)
DNS Addressing - How to Change in Windows 7

Thank you so much indeed for your continual replies.

On the internet, there is a very wide variety of tutorials for how to change DNS settings. However, what are very important for me are these points below::::

1- according on what we enter sequence in the preferred DNS server boxes since I think here might be the mistake.
In that guide you have linked is said:
In the Preferred DNS server boxes enter this sequence: 208.67.222.222

In the Alternate DNS server boxes enter this sequence: 208.67.220.220

2- Do you think that obtaining DNS sever address automatically is much better than manually use specified DNS server addresses? If so, or even not, then if I have now set manually specified DNS sever addressees while I no longer have any problem with my network with automatically DNS server addresses, then the performance would be much higher.

3- One more question about that tutorial you have linked its link is why that publisher changes 'DNS server settings' of a Local Area Connection instead of a Wireless Network Connection, although it looks from the screen shot that s/he is connected to a Wireless Network Connection and not a Local Area Connection.
I am expecting that I must be selecting the adapter to which I am connected to change DNS server settings.
In my case, I am always connected wirelessly to my Wireless Router. And sometimes I get connected via a wired cable to my router. So, I think, in my case, I must select 'a Wireless Network Connection'

4- You said below "Google websites are not more reliable, it may have been that your DNS (ISP assigned?) server had issues with other DNS servers". So, You meant that my ISP who assigned Google websites to be not affected by that DNS server problem. If so, who is my DNS server? My ISP is YemenNet corporation.

Google websites are not more reliable, it may have been that your DNS (ISP assigned?) server had issues with other DNS servers. Some reading https://en.wikipedia.org/wiki/Domain_Name_System which all start with the invisible dot (.) at the end of whatever we write in browser https://en.wikipedia.org/wiki/Root_name_serve