In this case the blockchain is stored in:
C:\Users\{YourUserName}\Appdata\Roaming\Bitcoin
As indicated here:
https://en.bitcoin.it/wiki/Data_directory
Go to Start -> Run (or press WinKey+R) and run this:
explorer %APPDATA%\Bitcoin
Bitcoin's data folder will open. For most users, this is the following locations:
C:\Documents and Settings\YourUserName\Application data\Bitcoin (XP)
C:\Users\YourUserName\Appdata\Roaming\Bitcoin (Vista and 7)
"AppData" and "Application data" are hidden by default.
Remove that Bitcoin directory, and you will have removed your wallet file and your blockchain.
Note, that if you don't have a good backup of your wallet.dat file, then removing this directory will result in the permanent loss of any bitcoins that you currently have in this wallet.