Your operating system isn't enough to tell us how fast your computer is. Nevertheless, Flash is still probably your best bet if you can afford it (although use the free trial for the first 30 days, of course). If not then I recommend Easytoon for learning how to redraw consistently. Or if you're lazy like me then Pivot is the way to go. Either of those last two will run on any computer that uses electricity. Flash might run slowly if your computer's old, but it'll probably be fine.
