The fsutil command line utility has a number of useful diagnostic and configuration tools for the Windows filesystem. Making a file of an arbitrary size, possibly very large, with this utility is very easy. Making a 1GB file with other methods (Wordpad, perhaps) can be tedious.
To create a 12000 byte file called file.out, use this command from the command line:
fsutil file createnew file.out 12000
To create a 10GB file, use the following:
fsutil file createnew bigfile.out 10737418240
The fsutil command has been maintained through all versions of windows including XP, Vista, and Windows 7.