Sometimes you just need a file with nothing particular in it so you can test a web-page with a <input type="file"/>
Rather than write a snippet of code, or use a utility, you can use the operating system to do this.
On WindowsXP, you can use FSUTIL to do this.
FSUTIL FILE CREATENEWFILE <path> <sizeInBytes>
FSUTIL FILE CREATENEWFILE C:\20mbfile.txt 20971520