If the original desktop has USB-1 ports, it's probably quite a bit out of date for some other specs as well. Starting from scratch might be about as sensible as trying to do a lot of upgrading. Going from Win98 may be something of a "culture shock" too, since for sound you'll probably want at least 250 GB hard drives and 2 GB of RAM.

Take a look at ExtremeTech and click on the "Build It" tab. Look at a few of the examples, and you might find something that fits your needs better than an upgrade. They put together a "sound system" (recording studio?) machine that looked pretty good a few months ago, although I don't know if it's still on the front page listing; and it did have a lot of bell$ and whi$tle$ in it.

If you get into newer machines, you'll likely find most offered with SATA hard drives rather than EIDE. The newer ones offer a little more "flexibility" than the old ones, but it's difficult to mix the two types in one machine. Even WinXP is now in "limited support," and most new computers (the cheapest ones, at least) often don't offer it as an option, so you could be stuck with Vista.

There's been much more market emphasis recently in computers set up specifically for "multimedia" - almost to the point that it's hard to do much traditional "computer stuff" on the new ones. You might find something reasonably budget-compatible in a new computer, with some of them selling for about 4x(?) the price of a good new hard drive.

All sorts of possibilities. One can worry so much about doing the "right thing" that nothing ever gets done.

John