Question

delete activities
where unt_uid is null

would be the fastest way but nobody can access the database / table until this statement has finished so this is a no-go.

I defined a cursor to get this task done during working time but anyway the impact to productivity is to big.
So how to delete these record so that the normal use of this database is guaranteed?

It’s a SQL-2005 Server on a 32-bit Win2003. Second Question is: How Long would you estimate for this job to be done (6 hours or 60 hours)? (Yes, i know that depends on the load but assume that this is a small-business environment)

Answer 1

You can do it in chunks. For example, every 10 seconds execute:

delete from activities where activityid in 
  (select top 1000 activityid from activities where unt_uid is null)

Obviously define the row count (I arbitrarily picked 1000) and interval (I picked 10 seconds) which makes the most sense for your application.