This query will detect any Windows 7 Computers and add them to the collection group:

select
SMS_R_SYSTEM.ResourceID,SMS_R_SYSTEM.ResourceType,SMS_R_SYSTEM.Name,
SMS_R_SYSTEM.SMSUniqueIdentifier,SMS_R_SYSTEM.ResourceDomainORWorkgroup,
SMS_R_SYSTEM.Client from sms_r_system where OperatingSystemNameandVersion like '%Workstation%6.1%'