I've seen posts about this, but fortunately never encountered it myself. The solution seems to be to either disable hardware acceleration or to change the Cleartype settings in Windows, with disabling acceleration being the easier of the two.
Go to Settings and first click the checkbox to show advanced settings. Then under Browser > System (or type "accel" in the search box) clear the checkbox for "Use hardware acceleration when available". I think you need to restart Opera for the change to take effect.