I went through this too building my wife's latest machine. Wanted something to make sure her data was secure.
Windows 7 has software mirroring but it has to be set up from the original install, we were moving her hard drive to the new machine so that was out.
I then did a search for "real time backup" and found a few options. I ended up with this:
You can set it to mirror whatever files from your main drive to a second, third or however many drives. It's not true mirroring but it will save your data. Any time you modify or create a file, Yadis! copies it to the backup drive. It has to be set to delete saved files when they're deleted from the main drive.
I installed this on my laptop too removing the cd drive and installing a second hard drive. Doesn't affect batter life noticeably.
All the Best,