As a fairly devout pc and electrical enthusiast ill give you my personal opinion.
fF it's been given to you its most probably pretty worthless anyway, running Vista is a horrific O/S doesn't help either.
Options:
Use the hard disk in your newer machine. (not really recommended)
How old is the disk, which brings us onto, how reliable is it? Ok all good to use it as a temp/swap/junk drive, but dont go making it an expansion to your valuable photo's and movies to find it dies 2days/weeks/months later.
Use the pc as a media device. (not really recommended)
There are many devices now on the market down to the size of a USB stick than can run and decode HD, something this pile of may not even be capable of.
ON top of that there's effiency, running the old switch mode PSU to use this old PC to do something relatively CPU intensive for something it wasn't designed for will most probbaly cost you more in electricity paying for that device you should of bought in the first place.
The only thing id say it's really good for is someone who can't afford a newer pc at the moment but is saving for something better, an interim solution. OR wiping it and installing an open source firewall proxy or if you're lucky add mutiple drives and build it as a dedicated NAS.(network attached storage)
Sorry to sound so negative. But ive been there and have the graveyard of many old pc's etc to know what is and isn't worth it!