lexipixel - 12:28 am on May 25, 2011 (gmt 0)

Grep this (or have the hosting company do it if you don't have shell access).

grep "eval(base64_decode(" . -rl | xargs sed -r -i -e '/<\?php \/\*\*\/eval\(base64_decode\('.+?'\)\); \?>/ {d}'

(DISCLAIMER: I did not write that code - it was supplied by someone more skillful at the prompt than I am. I can not decide it for you -- but can tell you it found the buggers).