I've had google luck running the Microsoft provided VPC (as described here http://www.russellheimlich.com/blog/walkthrough-setup-multiple-ie-virtual-machines-on-a-mac/) under VirtualBox.
Back when I did it, the tricky part wasn't running one of these VPC, but the second one (some VHD id duplicate issues if I recall well). Very quick, launch instantly - I stopped using Parallels once I had that. -- Thibaut
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