Hang on--in what way do they lower your score? Is it just because they're closed, or are there problems on them? Don't spring-clean until you know where you're going.
Old closed accounts help your overall length of credit history. Even after you close them, they stay on your report for 10 years after the date of closing. If there are "baddies" on them, they do stay on for 7 years from the time they happened (10 years in the case of bankruptcies), but their damage fades with time. If the baddies were relatively minor and several years old, you may be getting more benefit from the age of the account than damage from whatever's on them.
There are some cases where the vanishing of a closed account is a blessing, but they are surprisingly rare. Others will chime in her soon, but in the meantime, read this:
I'm on Firefox, so this doesn't show as a link. You'll have to copy/ paste it into your browser.
* Credit is a wonderful servant, but a terrible master. * Who's the boss --you or your credit? FICO's: EQ 781 - TU 793 - EX 779 (from PSECU) - Done credit hunting; having fun with credit gardening. - EQ 590 on 5/14/2007