I have intermittent issues with restarts and shutdowns, too.
Booting to the installation disc (if that's what you're referring to) can be done by holding down the C key during the bootup process or the Option key during the bootup process.
As G-Com states, pop your install DVD in and reboot holding down the Option key and see if you can get to the DVD that way. If any joy go to Utilities > Disk Utility > and run Repair Disk and see what is reported.
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