Boot from the install disk CD by inserting the disk and hold down the C key during boot, this should force booking off the optical disk. Then you can run hardware test utilities (Disk Utility). There are some other suggestions from:
http://docs.info.apple.com/article.html?artnum=106464
You see a blank, gray screen
1. Make sure the latest firmware for your computer is installed. Check Apple Downloads for the latest versions.
2. Disconnect all peripheral devices, except for the Apple keyboard and mouse. This includes ADB, serial, USB, FireWire, SCSI, and PCMCIA devices. If this resolves the issue, connect one device at a time, restarting after each one, until you've determined which device is causing the issue.
3. Remove third-party hardware upgrades such as memory (RAM), microprocessor upgrade cards, and PCI cards. If this resolves the issue, add back one item at a time, restarting after each one, to isolate the issue. Note: Apple does not provide technical support for Mac OS X when used with third-party microprocessor upgrade cards.
4. Start up from your Mac OS X installation disc and check the hard disk with Disk Utility or, if necessary, fsck.
5. If you are comfortable doing so, reset your computer's Power Management Unit. Specific directions are available elsewhere in the Knowledge Base.
6. Additional tips may be available in this document. (there is a link from the apple page to other similar tips).
