ColumnSet cols = new ColumnSet();
cols.Attributes = new string[]{"systemuserid"};
QueryByAttribute qe = new QueryByAttribute();
qe.Attributes = new string[] { "domainname" };
qe.Values = new string[] { "the_user_login_value" };
qe.ColumnSet = cols;
qe.EntityName = "systemuser";
BusinessEntityCollection bec = service.RetrieveMultiple(qe);
Guid userId = ((systemuser)bec.BusinessEntities[0]).systemuserid.Value;
My blog : http://mscrmtools.blogspot.com
You will find:
Bulk Delete Launcher(Delete data based on advanced find queries)
Form Javascript Manager (export/import javascript from forms)
ISV.Config Manager (graphical ISV.config edition - export/import)
View Layout replicator (customize one view and replicate to others)
And others (use tool tag on my blog)