Sharepoint Central Administration
Virtual Server Configuration
Configure virtual server settings
Default Web Site
Virtual Server Management
Define managed paths
Add New path -> certsrv -> Excluded path
OK
See: http://www.microsoft.com/technet/prodtechnol/biztalk/2006/library/a4swift/39916663-b80e-49d8-ba9b-49276eb564fc.mspx?mfr=true