Curser moves to search bar when typing, & other erratic keyboard and curser

I'd really love some help. My keyboard on my laptop is acting erratically. The cursor pops up to the browser search bar and types from there, and I need to bring the curser back down (both w firefox and chrome). If I'm in office it pops up to the MS Windows search bar. Before the backspace didn't work. I also had problems with scroll before. I've tried an external keyboard which eradicates the problem, which implies it's hardware and the keyboard. However, I've cleaned my laptop with Ccleaner, Rkill, malwarebytes, adwareblocker, hitman pro, and my usual antivirus is node. Hitman pro and adware B showed viruses (it's possible false positives). There was some improvement after cleaning my computer which implies viruses. At times my keyboard works for a few sentences, my keyboard could be working or it could be a short, I don't know. I turned off sticky keys and the other options like that.  I did a restore to before the problem was occurring.  Some of these things may have helped a little, or maybe not, the problem seems to be really bad at times, other times I can type a little more.  One last piece, 1.5 years ago I sat on my laptop when we didn't have light. It worked for a while then stopped. I needed to have the motherboard replaced, and it's worked for a year after that. I'd love some feedback on possibilities and ways to proceed. It's a dell i3542.  Windows 8.1, 500mb, 4 ram, i3 intel, 2 years old, as close as i remember for all that.  One more thing, I changed my default viewing for videos all to VLC a little before this happened.  I was having problems downloading and watching a work video (It didn't work well, VLC would freeze up).  This may be relevant or not.  But I'm guessing the restore would have corrected that if it was a problem.

Thank you for any suggestions, feedback, information.  And right now I've just typed a full paragraph.  10 minutes ago I could only type one or two keystrokes.  I don't know.  Also, will the antivirus programs like hitman, adware, etc., clean my usb's if their plugged in?  I don't want to corrupt my USB's if their clean, but I also don't want to corrupt my laptop if the usb's have viruses.

This also happens with my computer. I'll try to clean it by using combofix, adware  and Malwarebytres.

Sitting on laptops, like dropping them, is contra-indicated !  If you sat on it hard enough to damage the mobo then it is more than likely that there was at least some delayed damage to the keyboard - possibly a crack that has grown.  The good news is that these keyboards are quite easy to replace, see this video :-

They are not even expensive. Somehow I found myself on the Amazon US web-site and they are around $US 15 .

Chris Cosgrove