Cursor keeps moving down

Dear Friends,

I am in computer technlogy since last 15 years and i know desktop assembling.
and i am also vba excel programmer.

i am facing one strange issue on my laptop.
whenver i press "A" key on laptop keyboard.
webpage scroll down and then never come up till i press CTRL key.
or while working in excel if i press "A" kye my cursomer keeps move down some time i feel that "A" is doing task of PgUP or PGdn key.
or after pressing "A" character if i click on computer clock one calendar opens but those Monhts starts changing and when i press CTRL key it stops.
even i faced lot of trouble while drafting this query whenever i press "A" character
in short after pressing "A" i cant do any task because my cursor moves down.
my laptop is acer aspire 5755.
Pls help.