If you do go the mountain bike route, go for something with a rigid fork like this. You don't want a suspension fork on a new $500 bike, whether it's a mountain bike, a hybrid, or . . . something else.

Also, one of these would be a nice addition to that Windsor (or anything else with a six-bolt disc hub in the back). A fixed-gear rigid 29er with big fat tires is a lot of fun for city cruising. I have a Gary Fisher Rig (with a Redline steel fork) set up that way right now, and it's a great time. Definitely no Mark V, but it has its strong points.