US International Keyboard

I work with accented caracters all the time and my
keyboard is configured as US International. Three weeks
ago, something weird started happening. The acute sign
doesn't work with vowels and everytime I hit the (') key,
it shows: '', making it impossible to accent letters. The
other signs (~, ^) don't work at all.

I already tried changing the layout back and forth with
the US Standard, without success. Any tips on what I
should try next?

Thanks,

Daniela