Hey there!
The process to migrate data from a Windows computer to your new iMac is done by first having an External Hard Drive that can be recognized on both the Windows PC and the Mac, and using the Migration Assistant utility for Windows ( links below ) and for Mac OS X (pre-installed) and then connecting your External HD to the iMac, and using the pre-installed Migration Assistant tool (found in the Utilities folder in Applications) to bring said data on over from the External!
Here are some links to get you started:
About Windows Migration Assistant-
http://support.apple.com/kb/HT4796
Migration Assistant for Windows:
http://support.apple.com/kb/DL1557
How to use Migration Assistant (Mac)
http://support.apple.com/kb/HT4413
Here is a quick rundown:
1- Connect your External HD to the Windows computer
2- Download and install Migration assistant for Windows (read the article for info about what gets migrated)
3- Follow the Migration Assistant for Windows on-screen information to successfully set up the External drive with your files
4- Eject your HD
5- Connect it to the Mac
6- Open Migration Assistant ( Finder -> Applications -> Utilities -> Migration Assistant )
7- Follow the directions in Migration Assistant to migrate your data over
Good luck! Let us know if you have any questions or issues.
- Captain