Most likely cause is a bad Hard Drive.
Second most likely is a dirty CD, or dirty lens in CD-Drive.
Or failing CD-drive.
Sick RAM can cause that.
Overheating CPU, due to improper mounting of fan.
Failing Power supply.
Easy quick test is to boot up a small version of Linux off just the CD-Drive.
It can run even with a bad hard drive. If it has a hard time mounting the Hard drive and reading the directory, it becomes obvious it is the hard drive.
Here is an article dated 2003 from IBM:
http://www.ibm.com/developerworks/linux/library/l-knopp.htmlThe author says that Knoppix Linux on CD is a good tool to have.