Here's a link to a good list of PC shmups. Most are verts (of those, I'd recommend Cho Ren Sha 68K, Warning Forver and Noiz2sA.) but there are a few good horizontals in there as well (probably the best one I've seen is Bike Banditz.) Note that almost all of these will be in Japanese, and a couple may not even run on a non-Japanese PC.