Well, assuming you are talking about an Alesis Multimix Firewire then drivers are available for Vista from their web site. Reading the blurb there is suggests that the mixer came bundled with Cubase LE - is that how you remember it? Did you register the s/w with Steinberg because you should be able to find the relevant download somewhere on their site which you should be able to authorise from your account. Full versions of Cubase used a dongle to authorise but I think the LE version used an authentication code.
This web site seems to suggest that you can authenticate trial versions which you may feel is worth a go. Also have a look at
Steinberg's support site and look under 'unsupported products' which I think you will need to be compatable with Vista - I saw LE4 in there which should be compatible.
How many channels are you going to run? That is the main factor to consider if an old PC will do the business. If you are only recording - say - 4 tracks of real instruments then it should be fine. Main problem comes when you start running large numbers of virtual instruments with effects and processors tagged in. Probably OK to start with. If you can't get Cubase set up, have a look at Audacity which is free and will get you started. After that a troll round the digital audio workstation s/w vendors may find you some deals on either free 'light' versions, trials or etc. I can't really recommend any as I only use Cubase and Ableton. I have heard good things about
Reaper - it's available as a free trial and if you like it then a personal hobby licence is $60. I do suggest you start off with something free so that you get your eye (or should that be ear) back in again and you have sorted all the gremlins out of your system so you don't waste your trial period sorting out probs.