Mac OS 10.5.7 startup problem

[S0ULphIRE]

The apple changes to a 'no access' sign, like the ones over cigarettes in no smoking areas etc.

Any ideas? I'm drawing blanks as to what's happened. Before this the Mac just locked up, so I restarted and I get this sometimes. Other times it'll just stay on the spinning wheel.

 

[Kharn]

Clear the pram, but run the memory diagnostic thats on the OSX disk.

 

[S0ULphIRE]

What memory diagnostic? I can't see it on the install disk. I've booted off it, clicked on the 'utilities' dropdown menu, but it ain't there...

edit: Tried the pram reset, unfortunately it didn't fix the problem. When I boot into single user mode and do a fsck it always hits an error, says something about something's meant to be 16 instead of 17. I'll get the exact error.
edit: Incorrect block count for file pcscd.pub (it should be 16 instead of 17)
The above error came from trying to verify the disk off the boot cd. A repair apparently 'fixes' it, but obviously it doesn't, because the mac still doesn't start and the next time I try verifying the disk the same error happens.