If there were an easy way of doing that that didn;t look terrible, windows would probably already be doing it
In fact it is, basically you can't do too much except spline interpolate up which smoothes out the jaggies some but also blurs more.
You could load the low res icon into photoshop, enlarge, then redraw on top if you have some PS Skilz
Well ugh, photoshop doesn't do icons
some other icon editor...