How are you opening that website?

If you open it via a bookmark then the site should open in the current tab unless you middle-click or otherwise force it to open in a new tab.

If you are referring to opening in the sidebar then you can check the Properties of that bookmark and remove the check-mark to open in the sidebar. In the Bookmarks Manager you need to click the More button that shows below the Details pane at the bottom of the right pane.

No I didn't express myself clearly. The problem is not that the website opens in a new tab - but that it opens AUTOMATICALLY - that is I haven't tried to open it.

What website is that? Was it a pop-up window?

You can do a malware check with a few malware scan programs.
You need to use all programs because each detects different malware.
Make sure that you update each program to get the latest version of the database.

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

See also "Spyware on Windows": http://kb.mozillazine.org/Popups_not_blocked and What to do when searches take you to the wrong search website

It is not a pop-up-window. It is a tab. The website is https://www.hoeje-gladsaxe.skoleintra.dk/Infoweb/Fi2/Default.asp