You should do an upgrade to the current version of phpbb using the full package install version so that you will overwrite any changes the hackers have made. To get the admin logins back you will probably have to manually change the db using SQL commands. The phpbb_users table is the one you want to look at the closest for any changes. You should regularly upgrade your phpbb as there have been a lot of security bugs fixed over the last year.
It’s worse than that. There were also 7 security bugs found and fixed in PHP itself recently. So you’ll not only have to upgrade phpbb to 2.0.11, but also upgrade PHP to 4.3.10 or 5.0.3 also. Annoyingly there is also a bug in these versions that causes PHP to fail to compile the zend_strtod.c file on several platforms without manually patching the source.