Scroll bar drops to the bottom

I would be so grateful if anyone could help with this. I use XP SP2 and Firefox. My anti-malware is Avira, Malwarebytes,SpyBot and Panda anti-rootkit, all full and upgraded frequently. I suspect malware has been removed and left behind some changes, but that's just my guess.
Often - not always but usually, I find the vertical scroll bar drops to the bottom, showing the end of the window and making the programme unusable. It may not do this straight away, maybe after two minutes, maybe twenty. It affects Firefox, my usual browser, IE, which I also tested, Word 2007 and some folders - but not all. Sometimes it can be halted by re-starting. sometimes it goes away by itself. It makes no difference whether I am on-line or not, nor if I disable my connection.
When I began to type this in your window, after a few letters the typing was entered at the bottom of the windown - even though the scroll bar had not yet dropped. I have had to type with Notepad - which has once or twice been affected by the problem itself - and copy into your window.
While I suspect malware, I don't recall anything that looks nasty being found - mostly it was adware. All my anti-malware programmes have been run - and run again - finding nothing.
Any advice or assistance would be so welcome, especially if it solved the problem.