I second TrueCrypt as a free solution. We use that at work and it is great!
Last Question: How strong is BitLocker protection in case of physical theft of the usb stick??
BitLocker offers 128- and 256-bit AES encryption. TrueCrypt gives you a bit more options, but that would definitely be sufficient. However, to use BitLocker, you'll need a computer running Windows 7 Ultimate or Enterprise to perform the initial encryption.