It's a good thing you are not using WinXP, since the support and updates for that OS will stop this year, but Windows Vista is also not a good choice. It's about the worst Microsoft OS since Windows ME, even Microsoft admitted to that. So if you're upgrading, it's probably a good idea to upgrade your OS as well. A lot of people would recommend to stick with Win7 because of the Metro interface in Windows 8, but I've used Windows 8.1 for more than a year now and in combination with Classic Shell (like Tony stated), there's very little difference between it and Win7. (and you won't ever see the Metro interface, just your trusty desktop).

This might be a good opportunity (excuse? ) to do an upgrade after 8 years. Everything will be much snappier and you'd get some new features that you'll probably love, like eg. USB 3.0, which is more than 10 times faster than USB 2.0 (I know from older posts you use USB a lot)

I would advise you to get a completely new system though, because any hardware you'll transfer from your old system will probably either not work or slow the new system down. The motherboard obviously will not work, neither will the CPU and memory. The only things I can see working here are your power supply unit (which I would replace anyway after eight years) and your harddisk (assuming it's a SATA model and not IDE). But even then, I would use a new SSD harddisk as a startup/OS disk, accompanied with a new SATA harddisk for storage, and next to that, the old harddisk for additional storage (or backup even - of course, you'd still also always need an off-site backup too).

Putting a PC together yourself is really not that hard anymore these days. Most of the components reside on the motherboard nowadays, even the video card (an on-board video card is sufficient if you're not a hard-core gamer). The only things you'd still need to add are the CPU, the memory and the harddisk. Then connect the power supply unit to the motherboard and you're done. All the connectors are protected these days, so you can't even connect them the wrong way round even if you wanted to, so don't worry about that.

I'm pretty sure you would be able to put a completely new system together for about $600 to $700, including an SSD harddisk and a new Windows 8 licence.