[Xenon]

run repair.php more than once, sometimes it helps...

if it really doesn't work, delete the table and recreate it:
[sql]CREATE TABLE attachment (
attachmentid smallint(5) unsigned DEFAULT '0' NOT NULL auto_increment,
userid int(10) unsigned DEFAULT '0' NOT NULL,
dateline int(10) unsigned DEFAULT '0' NOT NULL,
filename varchar(100) NOT NULL,
filedata mediumtext NOT NULL,
visible smallint(5) unsigned DEFAULT '0' NOT NULL,
counter smallint(5) unsigned DEFAULT '0' NOT NULL,
PRIMARY KEY (attachmentid)
)[/sql]