Fastest way to update 120 Million records

I need to initialize a new field with the value -1 in a 120 Million record table.

Update table
       set int_field = -1;

I let it run for 5 hours before canceling it.

I tried running it with transaction level set to read uncommitted with the same results.

Recovery Model = Simple.
MS SQL Server 2005

Any advice on getting this done faster?