It depends upon what you plan to do with the program. Apart from the many limitations Prime (fulll version) has compared to Mathcad (and I have versions 14/15 in mind, but I guess a lot would apply tp 2001i, too), Prime Express is very limited. No symbolics, no programming, no solve blocks, only xy plots (no polar, 3D or contour plots) and most of the functions on "Functions" ribbon will be locked, too (Curve fitting, Data Analysis, DoE, Differential Equations, Image Processing, Probability Distr., Signal Processing., Statistics, Fourier transforms and who knows what else).
So I guess your 2001i might be the more valuable choice if you can get it to run. You have to giv it try if it works with some kind of compatibility settings or maybe you can setup a virtual machine runnnig an ancient Windows (98?).
But as Prime Express is free you can install both and give Prime Express a try anyway. But be prepare that you cannot exchange worksheets between the two. Prime3 is not able to save in any other Mathcad format than Prime3 (not even in Prime2!) and it is also not able to read Pre-Prime files. You can convert older files to Prime format, but you need a full installation of the current Mathcad 15 (there is a 30 day eval version availavle) to be able to do so.
Furthermore you have to consider that with 2001i most of the time you won't be able to open Mathcad files you find on the net (but then thats true for Prime as well as most files you find will be in the range MC11 to MC15).
