Dns dynamic update problem, and scavenging

Hi!

I have 2 days of lease time. How can i calculate the scavanging period and refresh, no-refresh days?

I dont understand how refresh no refresh works. Somebody can explain it clearly?

My second question is:

Should I use DHCP server for deleting the expired records?

My problem now, that old records remains, clients not updating dns records. I use the "Dynamically update DNS A and PTR records only if requested by clients", only secure update. Domain enviroment and Clients are XP and Windows 7. Because of this problem, i thought i turn on scavenging. Is this ok for my problem?

Thanks,!