I tried another one and it's OK.
I've not installed any new programs or made any changes, but now when I am on a web page & arrow up it goes to the top of the page, arrow down goes straight to the bottom of the page instead of scrolling. I've tried the "scroll lock" key, no difference.
The Page up/down key sometimes works, sometimes not. I'll try clicking different parts of the screen & can get it work sometimes, but usually just go to the right margin & drag whatever that's called to scroll up & down a page.
Windows Vista, Lenovo computer, Dell keyboard.