Source: PlanetHalfLife

A fix for the crosshair problem some have with Half-Life 2, a Q appears in the crosshair, has been released by BullseyeCrosshairs.com. The fix/replacement is currently only for the single player:

While trying to solve the mystery of some Half-Life 2 players seeing the letter Q appearing where the crosshair should be, it was discovered that the tiny 5 white dots were being generated by a HL2 font file. With a little manipulation using a true type font editor, it's possible to alter the design. Bullseye Crosshairs has it's first replacement up for download.
Unfortunately it only works in the single player game for now.

You can grab the replacement here.