Well there could be several things wrong. First off when you boot it up, is there any beeps? There are beep codes for your bios. www.computerhope.com/beep.htm
Could be your RAM went bad, there is a way to test your RAM with your CDROM. http://oca.microsoft.com/en/windiag.asp
If you have a meter you can test your votages on your power supply, but does not sound like a power supply problem.
Now about saving your data. Your computer is five years old. You are really out of date. I don't know what you use your computer for, but if it is for games or business, go invest $600 and get an AMD Duo Core 2.0 or higher system, with at least an 6000 or 7000 series nvidia video card. If you got vista you will need 2 Gig of RAM.
Some RAM has a lifetime guarantee. Take a look at your RAM and see who the manufacturer is.
Now as far as the hard drive is concerned, most likely your data is fully secured. All you need to do is to make it a secondary hard drive. If you check Hard Drive only have a life of five years. So probably your Hard Drive has errors on it. That what it really sounds like to me. So if you purchase a new hard drive (www.tigerdirect.com) and put it in and use your restore disk and load up the system and drivers. Now that your are up and running, just load your data on the old drive. Do a disk error run on that drive. Go to my computer, right click properties, tools disk error, check the box and reboot. Do this after you transfer you data. If it is bad, then take a large magnet and put it over the drive and throw it out. I completely destroy my harddrive and take out the disk and magnetize it.
Hope all this helps.