Try updating the prebinding, or "optimizing system performance" as Apple call it.
Go to the terminal and type
"sudo update_prebinding -root /"
without the quotation marks, of course, then enter, type your password and press enter again.
You can also try to start up in single user mode, (hold down command and s while booting), then type
"fsck -y"
into the console, and press enter when it's finished booting. That will check and eventually repair the startup volume catalog file. Then type "reboot" and enter when it's finished to boot up normally.