-snip-
cd C:\Users\YourUserName\Appdata\Roaming\Bitcoin
In Windows, it's usually in C:\Program Files\Bitcoin but may be different depending on where you installed Bitcoin.
Another way to launch it without "cd" is to provide the absolute path to bitcoin-qt like: "C:\Program Files\Bitcoin\bitcoin-qt.exe" -reindex
Another option is to create a shortcut of bitcoin-qt and in its 'Properties', edit the "Target" with the command line option that you want to use.
e.g. (the shortcut's "Target"): "C:\Program Files\Bitcoin\bitcoin-qt.exe" -reindex
Click 'Ok', Then use that shortcut to start Bitcoin Core with -reindex.