Last in the list opens every time

hello,
i have a dell studio 1535 laptop with win 7 os,500gb hdd,intel core duo processor....my laptop gone berzerk....after the startup,as usual desktop is appearing and when i click anywhere the last shortcut icon in the desktop is opening everyime even when i try to open another icon in the desktop...not only the desktop even the files in a folder or in a drive or when i search for something in th search bar the last option gets selecting automatically......and when i open any website, it is going to the bottom of the page and not going up by scrolling it...i have to use the pageup key in the keyboard to go up....it has ben like this for a month...even the antivirus scan is no use..........can you please show me the solution for this..???it is so irritating!!
thanks in advance