Disable the TCP Chimney on all of the domain controllers
2003 Servers
Netsh int ip set chimney DISABLED
2008 Servers
netsh int tcp set global chimney=disabled
netsh int tcp set global rss=disabled
After that reboot all of your domain controllers
Also your can run this command on all of the domain controllers to sync them
repadmin /syncall
Now try updating the schema first before installing Exchange 2007 SP2 or SP3
setup.com /prepareschema
Now you can run the setup like normal
http://support.microsoft.com/kb/912222
http://support.microsoft.com/kb/951037