FIXed IP and DHCP IP

Hi,

Just found a curious problem for my site-to-site VPN network:

1. Two remote sites ( less than 20 users ) connected to the HQ with site-to-site VPN Tunnels
2. Both remote sites use PIX506E V6.3(4) and HQ uses PIX515E V6.3(3)
3. Both remote sites used DHCP to distribute the TCP/IP setting to users, and the DHCP server is configured in the PIXes.
4. The second remote sites is located far from the HQ & the first remote site
5. The remote sites use WinXP professional, while HQ uses W2K3 AD domain.
6. The first site works perfectly.
7. The second site log onto the AD very slowly ( 10 times slow ) with DHCP setting, but works properly with fixed TCP/IP setting.

Anybody knows the reason, and how to correct the problems

Thank you Benson