在SQL 2005中删除聚簇主键时会发生什么

I've a PK constraint - a clustered index on two columns - which I am in the process of dropping. The command is still running after an hour. I would have thought that as I am just removing a constraint the operation would be nearly instantaneous. Can someone explain to me what is actually happening under the hood when I drop the PK?I've a PK constraint - a clustered index on two