from the description of the symptoms I'd assume this to be a failing hard drive. Given the age, its suspect anyway. Replace it.
As matter of interest, what make / size was the drive? I'd bet on an 80GB Excelstor, given what you've said, but the symptoms could happen on any
And by the way, throw that old Symantec install disk away. Symantec antivirus doesn't work, period. And old versions work even less. It certainly won't clean an infected machine