There are a number of ways that you can do this including:
Using os.path to join a path and an environment variable:
os.chdir(os.path.join(os.getenv('userprofile'),'Desktop'))
You could alternatively use either double backslashes (backslash needs to be escaped in Python strings):
os.chdir('c:\\users\\prosserc\\desktop')
or use a raw string:
os.chdir(r'c:\users\prosserc\desktop')
I would recommend the first option as it does require a hard coded user name.