Can you tell us more about your current system setup? What kind of PC are you using? What OS? Laptop or desktop? Do you have an external hard drive big enough to at least copy your entire PC's data over to it?
How often do you want this backup done - monthly, weekly, daily, hourly? Why do you want a full system backup instead of just the important data backed up? Are you anticipating a hard drive crash in the near future?
Sorry to bombard you with some many technical questions but there are so many ways to back up a PC that in order to help you evaluate them we need know what you are trying to do.
If you are like most people and want to have all of your data backed up in case something happens to your computer and you are often connected to the internet, then an online backup like Mozy will be a much better fit.
Also, Windows XP and Vista have built in backup utilities - you just need to tell it what you want to backup and where to put it. Software like Genie and Symantec Ghost can do a lot more, like make a copy of a disk without having to boot to the operating system, but for most people that's overkill.