You have either a failing hard drive or a failing power supply.
If you can boot the computer at all immediately copy all your documents, etc
to CD or DVD or whatever.
There are many software programs you can download to test the hard drive:
they are usually very rapidly accurate.
If the hard drive is failing it is not difficult to install a replacement.
There are many programs to "clone" the data on your old hard drive to your
new one. Some of them even work.
If your hard drive passes all tests, as I suspect it will, the problem is
your power supply. Most off the shelf boxes come with horrible power
supplies.
If you are not experienced at replacing the power supply and rerunning
associated cables then take your machine to a repair shop.