Another free encryption tool, FreeOTFE (On The Fly Encryption), has several features designed to make it particularly useful for situations in which you can't install the software: a "portable mode," which requires administrator access but no installation, and a separate but compatible (can read the same encryption) program called FreeOTFE Explorer that needs no drivers at all. Speaking of drivers, FreeOTFE will work on 64-bit Vista and Windows 7 systems, but because its drivers are not signed and thus run afoul of Windows security, you must jump through quite a few hoops to get them to work, most of which require disabling driver-signature verification.
All of the disk-encryption programs mentioned above support the SISWG IEEE P1619 standard, which is currently considered to provide a balance between speed and resistance to attacks based on tweaking data. In addition, many companies consider compliance with an IEEE standard to be a "checkmark" item when evaluating software. Each of the utilities supports other encryption formats as well; it's best to study your options and understand the strengths and weaknesses of each format, depending on your needs. For most users the default choices will be fine.
Set Your Data to Self-Destruct When You're Done With It
The final component of securing your data is making sure that any files you want dead are really most sincerely dead, and for this task you must turn to disk- and file-removal tools. Using the standard Windows Recycle Bin merely removes the visible reference to the file and marks the space as available; Windows does not truly delete any data until something overwrites that data, and may leave large chunks of recoverable data visible. Those leftover chunks allow undelete and file-recovery tools to work; the trade-off when you use strong file-removal tools is that you won't be able to restore accidentally zapped data easily, unless you've previously backed it up on another source.