Workaround worked for me
USB keyboard here, Microsoft Natural Ergonomic 4000. Same symptoms,
left and up arrows + the Del/Ins group got mapped to other stuff.
Plus, mouse buttons (including scroll wheel turns) emited extra stuff
with negative impact in some shooter games such as OpenArena and
Tremulous (any click made the character face straight down). The mouse
is also an USB mouse (Ideazon Reaper Edge). Also tried another mouse
(office Logitech USB mouse) and a no-name PS/2 keyboard.
FWIW, everything was affected the same way, so it looks like it's not
a matter of just-USB or just-keyboard/mouse, or just a specific
peripheral. Seems like a more global problem.
It's not a fix in the long run and we need to discover what's actually
wrong, but as a workaround Yuanle's solution is good.