need to block a specific site

For something quick to block the site and its links, there is "Blocksite", for normal stuff there is "Adblock Plus" and the filters might even already block it, though, but I won't test it. With ADP you must install filters such as "EasyList" once installed, and you want to add filters you make up yourself as well.

• BlockSite :: Add-ons for Firefox
  https://addons.mozilla.org/en-US/firefox/addon/blocksite/
• Adblock Plus :: Add-ons for Firefox
  https://addons.mozilla.org/en-US/firefox/addon/adblock-plus/

  my notes -- http://dmcritchie.mvps.org/firefox/firefox.htm#adblockplus

Thank you so much for such a fast and prompt answer!!!I'll install all that stuff right away!

Do a malware check with some malware scanning programs on the Windows computer.
You need to scan with all programs because each program detects different malware.
Make sure that you update each program to get the latest version of their databases before doing a scan.

• http://www.malwarebytes.org/mbam.php - Malwarebytes' Anti-Malware
• http://www.superantispyware.com/ - SuperAntispyware
• http://www.microsoft.com/windows/products/winfamily/defender/default.mspx - Windows Defender: Home Page
• http://www.safer-networking.org/en/index.html - Spybot Search & Destroy
• http://www.lavasoft.com/products/ad_aware_free.php - Ad-Aware Free

See also:

• "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked