Depends on what you mean by a "half decent aviation GPS package".
There is nothing out there which emulates a dedicated aviation GPS like a 496, with route planning etc features etc etc.
If you just want a moving map, Memory Map does the UK, and Oziexplorer does anywhere for where you can "acquire" the charts. Both these run fine on WM (pocket/pc) even in 640x480 on those PDAs which need the display hack to switch the display to 640x480.
But WM is a pile of crap. It crashes readily, and the device has to be reset when you start it up, just to make sure no garbage from the previous session is mucking up the memory.
Far better to get an XP tablet (e.g. Samsung Q1 Ultra) and run the above software on that, etc.