What to look for when installing DHCP on windows server 2008 that is also DC

Make sure DHCP is not running as a system account. This change is not necessary if DHCP is running on member server of domain.

By Using the DHCP Server Snap-In

1. In the DHCP Server snap-in, which is located in the Administrative Tools folder, right-click the DHCP server that you want to configure, and then click Properties.
2. On the Advanced tab, click Credentials.
3. Type the username, domain and password of the account under which you want the DHCP Server service to run. You can use any valid existing user account for this, such as a Domain User account. The account should not be set to expire or have any other restrictions.
4. Click OK, and then OK again to exit the Properties dialog box.

By Using the Netsh.exe Command Line

1. From a command prompt, type netsh, and then press ENTER.
2. From the netsh prompt, type dhcp server ipaddress (where ipaddress is the IP address of the DHCP server that you want to configure), and then press ENTER.
3. Type set dnscredentials username domain password (where username domain password is the user account information for the account under which you want the DHCP Server to run), and then press ENTER. You can use any valid existing user account for this, such as a Domain User account. The account should not be set to expire or have any other restrictions.
4. Type quit, and then press ENTER to exit.

Original KB article can be found at:
http://support.microsoft.com/kb/282001

Also very usefull article about DNS and DHCP correlation:
http://technet.microsoft.com/en-us/library/dd145315%28WS.10%29.aspx