You have report to list all computers based on last logged on username from http://eskonr.com/2011/03/sccm-report-to-list-all-computers-based-on-last-logged-on-user-name-from-collection/ .
In this post,you will get list of computers with last logged on user name from given collection.
Create a new report and paste the below query into it.
Select A.Name0, A.UserName0 from V_GS_COMPUTER_SYSTEM A join v_FullCollectionMembership B on A.ResourceID = B.ResourceID Where B.CollectionID =@collID
Create a prompt for collection ID collID :
select CollectionID,Name from v_Collection
Please refer the post why should u use V_GS_COMPUTER_SYSTEM for user name rather V_R_SYSTEM http://smsug.ca/blogs/garth_jones/archive/2007/06/30/482.aspx