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Mountain Lion includes a utility called Migration Assistant. You can use Migration Assistant (which can use Time Machine) to migrate your data. It will give you the option to transfer your data from the PC if you are on the same network.
I looked on MacWorld to see if I could find something about it, but the article I found on Migration Assistant doesn't focus on moving files from a PC specifically. Still, it guides you through the process, and hopefully will be of help.
Apple support makes it sound like a fairly simple process (not surprising, since it's obviously one of the first things a new Mac user wants to do). In fact, the company has created an app called Migration Assistant that comes installed on Macs. (You'll need to donwload it on your PC as well if you don't have it already.) After that, you just follow the step-by-step instructions on the linked support page.