Will a primary key be added as a clustered index? I've inherited a database where no primary keys were defined on the tables. There are also no clustered indexes assigned to the tables. 

If I perform an alter table to assign a primary key will this result in SQL Server also creating a clustered index? If so should I expect slowness on the database due to IO from the data being repositioned on the harddisk?
Yes, SQL Server will create the primary key as clustered by default, but you don't have to accept the defaults.

ALTER TABLE dbo.foo 
  ADD CONSTRAINT pk PRIMARY KEY (bar); -- clustered

ALTER TABLE dbo.foo 
  ADD CONSTRAINT pk PRIMARY KEY CLUSTERED (bar); -- clustered

ALTER TABLE dbo.foo 
  ADD CONSTRAINT pk PRIMARY KEY NONCLUSTERED (bar); -- non-clustered


And yes, you will see some I/O activity here, so if it is a busy system, best to save this for quieter hours or a maintenance period.