If that's a SATA computer, you should look into getting a SSD hard drive for it. You can then carry that drive on to your next computer. Makes a MASSIVE improvement to an older computer - I put one in an old Core 2 era Pentium laptop and it became tons more usable.
If you don't mind slow bootup and program opening and it's just slow when loading web pages, etc, then yeah, it's a problem I'm running into too with my older laptops...Internet browsers and web pages are getting a bit slow on chips from 5+ years ago. If that's the case, Linux would be the best way to get some more life out of that PC. It's much more efficient than any version of Windows.