Google produced this: https://www.google.com/#hl=en&sugexp=les%3Beaqth&gs_nf=1&gs_mss=microsoft%20security%20essentials%20slow%20s&cp=38&gs_id=1qi&xhr=t&q=microsoft+security+essentials+slow+scan&pf=p&sclient=psy-ab&oq=microsoft+security+essentials+slow+san&gs_l=&pbx=1&bav=on.2,or.r_gc.r_pw.r_qf.&fp=cd0820781f4eef2e&biw=1280&bih=887

Note that there is often additional details added by other users, might be helpful if this were noted here as well. You have a second antivirus or a trial version installed on your system?

Get WOT (World Of Trust) add-on for your browser if you do not have it. Much safer to visit websites than before. http://www.mywot.com/

Once WOT is installed, you have better choices on what is safe to visit.

How long did the scanning task take?