two options that make sense
1) use trucrypt
TrueCrypt - Free Open-Source On-The-Fly Disk Encryption Software for Windows 7/Vista/XP, Mac OS X and Linux
thats free
2) use the online upgrade feature of windows to switch to one of the Win7 versions that come with Bitlocker encryption - ie Ultimate or Enterprise
BitLocker Drive Encryption - Windows 7 features - Microsoft Windows