Please post screenshots of what you're seeing. Also, we need to know details of the kinds of footage, the version number of After Effects, and w
hat operating system you're using. More information will help us to help you.
Check your comp viewer quality setting and zoom level. The same applies pixel aspect ratio correction, al lof which affect the display quality. Aside from that, you may also simply see artifacts in the footage itself that show up after applying effects or changing Gamma and Exposure settings.
Sounds to me like AE is automatically setting field interpretation to your footage.
AE doesn't support fields in preview, only when it renders output, so odds are your footage has been automatically set to either upper or lower field interpretation. The result is that, during preview, you see an interlaced (half resolution) image.
Try temporarily turning off field interpretation, but remember to turn it back on before rendering (if the footage is interlaced).