Change typename in lookup ms crm

i have one challenge as follows, if you can....

on look up, onclick of it a popup window will come, there you can select a record.

there also a picklist will be there(sometimes) where you can also change. the best example is the regarding attribute on letter.

suppose on that picklist i have 3 options as a,b,c.

bydefault a is selected and on basis of some condition i want b to be selected.
its doable and as follows.

just put the following code on load of the form.

crmForm.all..defaulttype = "2";//the number of the picklist value, starts from 1


Sudhanshu