C:\drive (or from whichever drive you configured Windows to run).
Tip: the two other most common largest single files on a main drive will be
VHD virtual hard disk files for VirtualPC (like Windows XP Mode) and
hiberfile.sys, used when your PC hibernates.
pagefile.sysis the size of RAM detected by the OS, by default.
Tip: the minimum size should be at least
5% of detected RAM (example: 400Mb for 8Gb of RAM).