I've been meaning to do a write up on tools for applying patches on Windows but haven't had a chance. For now I'll just leave it at this list.

GNU Commandline Utilities

All of these can be downloaded as setup files that install into C:\Program Files\GnuWin32 (by default) so you'll want to add C:\Program Files\GnuWin32\bin to your system path.

DiffUtils - show differences between files
http://gnuwin32.sourceforge.net/packages/diffutils.htm

CoreUtils - GNU file, shell and text utilities
http://gnuwin32.sourceforge.net/downlinks/coreutils.php

CygUtils - miscellaneous utilities including dos2unix and unix2dos (which you'll need to convert the line endings to keep patch happy)
http://gnuwin32.sourceforge.net/packages/cygutils.htm

Patch - apply a diff file to an original
http://gnuwin32.sourceforge.net/packages/patch.htm

Wget - retrieve files from the WWW
http://gnuwin32.sourceforge.net/packages/wget.htm

GUI Apps

WinMerge - visually compare the text files
http://winmerge.org/