TrueCrypt also has a portable version.

But anyway, like the victim said, encryption alone isn't a solution. Encryption is nothing unless your system is guaranteed to be malware-free. Keyloggers can easily steal your password, which makes all the encryption in the world useless.

Use a secure non-Windows OS to store your wallet and restrict its exposure to the Internet as much as humanly possible (minuscule online time, aggressive firewall settings, aggressive anti-malware settings, aggressive antivirus settings etc.).