The primary key is enforced by an index and, I'd guess from your description that the primary key is the clustered index (use sp_helpindex to check). You can't drop the index that enforces a constraint without dropping the constraint itself and, with the primary key, that means dropping the foreign keys as well.

You should be able to drop the nonclustered index without problems. If you get problems, then post the output of helpindex on that table.

What are you trying to achieve?