Holy Doris Roberts -- old is the new chic. Alicia Silverstone and Christina Applegate (right) have donned retro frocks, and updated corsets are rearranging Catherine Zeta-Jones and 24's Elisha Cuthbert. ''Corsets are very old-time, but there's a younger feel to them now,'' says Cuthbert. Says Estee Lauder style guru Paul Starr: ''Things from 10 years ago can be considered vintage.'' What's next, grunge?