My friend have a problem with his Mac?

My Mac G-4 won't start up.?

• I have a G4 Quicksilver. Dual processor, I believe. OS 10.5.8. When I power it up, it shows the 'forbidden' symbol on a gray screen and it won't do anything else. I researched it and tried several different ways to remedy it. I unplugged everything but the keyboard and monitor. Then I removed the PRAM battery. No luck. I took the tower to my friend's workplace because he has the original OS CD. He plugged a regular monitor into it (not a Mac monitor) and a Mac keyboard/mouse. On power-up, it continued to show the 'forbidden' symbol. After putting in the OS CD, it started up as normal. We restarted it twice and it powered up correctly. We removed the OS CD and, again, it powered up normally. I brought the tower home and plugged in the Mac monitor and Lacie drive. Again, it went back to the 'forbidden' symbol on power up. Thinking maybe the monitor was the problem, I unplugged it and plugged in a monitor from a PC. That didn't work. With all of this info, are there any suggestions?

• Answer:

  LaCie drive? hmm, don't know if this could help but get that CD again and then go into system preferences and check the boot order. The selection should be the hard drive. If you do get it to boot up normally once then you should check out a free software program called Onyx. Download your correct version and have a ball. This is a great program that checks your hard drive and cleans it up too.