Modify the my.ini configuration file (assuming default paths)

C:\Program Files\MySQL\MySQL Server 5.5\my.ini

In the SERVER SECTION, under [mysqld], add the following line:

skip-grant-tables

so that you have

# SERVER SECTION
# ----------------------------------------------------------------------
#
# The following options will be read by the MySQL Server. Make sure that
# you have installed the server correctly (see above) so it reads this 
# file.
#
[mysqld]

skip-grant-tables

Start the service again and you should be able to log into your database without a password.