How to install exchange 2007 in child domain of Win 2k8

Hi pro,

 I've 1 PC (PC1) with domain: "test.com" installed exchange 2k10.

PC2 is child domain of PC1 with domain: "child.test.com". Now I want to install exchange 2k7 In PC2, but it show errors as:

http://postimage.org/image/4hoajmib1/

I read some infomations in Internet, they mentioned: "need replication PC" so, do you think I need 1 more PC (call PC3) which has role: "replication domain"?

So could you instruct me how to install exchange 2007 in Win2k8 step by step 

Thanks.