Much as I like the functionality provided by Parallels, it does have a nasty habit of hanging my computer. For me what usually happens is I click on it in my Dock and it starts bouncing... and then the bounce get slower... and then the whole computer gets slower... and then I have to force it to quit which takes a minute or so since it's so badly hung.

What I've found is that if I use Firefox while Parallels is starting up and bouncing in the Dock, it'll get hung. So for me the solution is simple. When Parallels is starting up and bouncing in the Dock I don't use Firefox or any other application. So long as I do this, it seems to start up much more reliably.