Adding and filling new column on big table

I have a table of about 500,000 rows.

I need to add a new column and populate it.

So, I have tried to run the following command. The command never finishes (I gave up after about and hour and a half!).

Note that none of the columns have indexes.

Update mytable set new_column =   

case when column_1 = column_2                                                                        then 1  

when column_1+column_3= column_2 and column_3 > 0                                      then 2  

when column_1+column_3+column_4 = column_2 and column_4 > 0                     then 3  

when column_1+column_3+column_4+column_5 = column_2 and column_5 > 0     then 4  

else                                                                                                                         0

end

My computer is a Pentium 4 – 2.4 GHZ and 1G RAM – so it should be fast enough.

Any ideas?

Jonathan Blitz