How do you apply "-zapwallettxes" on Windows?
-zapwallettxes removes all wallet transactions that are not in the blockchain. It can remove "stuck" transactions, although you don't need this feature in normal operations mode.

Solution A)

On Windows, you need to start the command line prompt (cmd.exe), go to your window search and enter "cmd" and start cmd.exe.
Start bitcoin-qt with --zapwallettxes=1

C:\Program Files (x86)\Bitcoin\bitcoin-qt.exe --zapwallettxes=1


(maybe adapt the program files path)

Solution B)

Create a bitcoin.conf file (normal textfile) here:

C:\Documents and Settings\YourUserName\Application data\Bitcoin (XP)
C:\Users\YourUserName\Appdata\Roaming\Bitcoin (Vista and 7)


Place....

zapwallettxes=1


... in the bitcoin.conf file, start Bitcoin-Qt, remove the zapwallettxes=1, restart Bitcoin-Qt.