Edwin,
I am on XP in Parallels and especially for using Quickbooks 2006, which i love and don't want to 'be forced' by Microsoft and especially Intuit to upgrade to Win 8 and a more recent version of QB.
So, I installed Windows7 in Parallels, which is still supported by Microsoft, and my QBooks works great on it. I am however, still on OS X Snow Leopard and from what i have read, have no plans to upgrade to Mavericks. I did, however, upgrade to Parallels 9, which runs slower so far, but i tweaked Parallels from what i have read here, especially in the 'Configure' menu under 'Virtual Machines" and that has helped. Before, Windows 7 was running sluggish, but is much quicker now. Hope this helps