My computer keeps reverting to finder

What I think you are seeing is the application is quitting on its own. Most of the time if you wait a bit you'll get an error message stating the application quit unexpectedly. Often this is a HD or OS problem. The drive's OS is corrupted or its starting to fail.

At this point I would start by making a backup of anything important onto a USB or FireWire drive just to be safe ASAP!

Now the hard part some how we need to boot up on a different drive so we can run drive diagnostics. One way is to use the OS install CD/DVD that came with your system. The other is to create a bootable FireWire drive. The next way is to connect a second Mac to your system via a FireWire cable we call this Target Mode.

You may need to install a new hard drive into your system are you up to it? As this is an older system think about how much you are willing to invest into it vs getting a newer system (New or Used).

Let us know what you have to work with here so we can give you the next steps.