no - the dice aren't loaded. (except when rolling for STAT's
) It's probably your imagination.
Critical Miss (d20=1) - always misses/fail and IIRC the character loses the reat of his/her attacks that round
Critical Hit (d20=20) - always hits/succeeds and the target takes double damage - unless protected by a 'helmet' (some monsters count as having helmets)
No STR bonus doesn't apply to bows - the DEX-modifier is added to THAC0 - damage depends on the type of arrow (no bonus)