I am considering getting Compaq  Presario Windows 98 operating system fixed, but need to find out what the problem may be before I take it into shop for repair. I had the hard drive replaced 3 years old. It won't power up, when I try to turn it on, there is a light that blinks in the back of the machine. What does this mean. It doesn't run though.  

Are you saying that the computer has no power or that Windows98 won't boot up?
Do the fans come on? Do you see anything at all on the monitor? Are there any beeps from the system speaker and if so, how many?
There is a beep code for various motherboards that can tell you what the boot up problem is.
If you hear no fans running or any other normal bootup noises, then you probably have a power supply failure.
If you see the BIOS displayed on the screen, you can use it to determine what the problem is.