User accounts alone won't protect your private data from someone determined to get at it. To make your hard drive snoop-proof, you need to encrypt it. The easiest way to do this is with Bitlocker, the full-disk encryption built into the Ultimate and Enterprise editions of Windows 7 and Vista, as well as in the Pro and Enterprise versions of Windows 8. By default Bitlocker isn't enabled, but you can turn it on by going to Control Panel > System and Security > Bitlocker Drive Encryption. Encrypting your whole drive will make it completely inaccessible to anyone who doesn't have your Windows user password.
If you don't have Bitlocker, you can still get free full-drive encryption with TrueCrypt, which secures your files with military-grade protection. Like Bitlocker, it can be used to encrypt your whole hard drive or just a subset of files. Files locked with TrueCrypt aren't just inaccessible--they're invisible to anyone who doesn't have the password for the containing volume.
Don't assume a deleted file is gone for good. Windows doesn't actually remove the data from your hard drive, it simply gets rid of the bit of your file system that points to the file. Even if you remember to empty your computer's recycle bin, trashed data can frequently be recovered with software like Recuva. There are many utilities available to destroy a file. A program like Eraser, for instance, actually overwrites that data on your hard drive, ensuring that it can never be resurrected.
Corral clandestine activities to a private OS
Many privacy pitfalls boil down to the fact that Windows is a complicated operating system, and it's difficult to keep track of all the nooks and crannies into which your data is being sorted. To eliminate evidence of your activities, the best option is to work in an operating system entirely separate from your regular one.
One easy way to do this is with virtualization. Using free software like VirtualBox, you can run a separate Windows or Linux computer in a window on your PC. Virtualization sets up a wall around the virtual computer, so nothing you do on the PC in VirtualBox can leave any files on your normal Windows file system. If you're really paranoid, you can even keep the VirtualBox data in a TrueCrypt volume, so that nobody but you can see it.
Finally, if nothing so far in this article is enough security for your needs, there's one last piece of software that provides absolute privacy. It's called Tails OS, and it's for when you want to go deep undercover.
Tails OS is part of the Tor Project, which we discussed earlier. It's not a web browser like Tor itself, but rather an entire operating system devoted to privacy. You install Tails OS on a DVD or USB drive and run it on any computer you want. Everything needed to run Tails OS stays on the removable disk, and nothing gets written to the computer's main drives. All data transmitted to and from the computer is filtered through the Tor network, so your browsing activity is completely untraceable, as well. Also, the operating system comes loaded with encryption and secure erase tools, as well as other privacy-oriented software. Think of it as your pocket privacy toolkit.
Following these tips is strong first step to reclaiming your privacy.