You could attempt to repair the hard disk drive and repair permissions, the OS X
may have damaged files; if the original hard drive is still inside there, they do wear
out and sometimes before failure could corrupt data on the drive. Issues result.
With the install-restore DVD, start the computer to access the Installer's version of
Disk Utility, choose first aid, repair disk. There may also be an Apple Hardware Test
on a disc, they were included in the original grey disc set. The AHT may not be all
that helpful, since there are times when an error or sector problem won't appear bad.
If the hard disk drive has little free or unused capacity, you should attempt to save
your personal files to an external drive; especially if the internal HDD may fail soon.
A third-party disk utility such as Disk Warrior (among others, each has a purpose)
may be able to help. Since I no longer use the older OS X that much, I have none.
Instead I relied on bootable system clones in externally enclosed hard disk drives.
If you have a second similar vintage Mac, the ability to use FireWire Target Disk
may help at some point. This can be used to help retrieve data from a HDD that
may not usually start the host computer, by using another Mac.
Perhaps someone with more or better ideas will see your thread & reply.
Good luck & happy computing! 🙂