Hi,
The dc7700SFF machines come with few models which normally use 45W CPU's. The E7200 requires 65W and the error simply means CPU not supported. You may want to upgrade the BIOS and try agian (not guarantee):
http://h20565.www2.hp.com/portal/site/hpsc/public/psi/home/?lang=en&cc=us&sp4ts.oid=3232029
Regards.