Back Up Your Data before Installing Windows 7
Before you jump into installing Windows 7 on your computer, you'll want to make sure your data stays safe. You can back up your files in a couple of different ways, depending on the Windows installation you currently have running.
Individual profiles are usually stored in C:\Documents and Settings for Windows XP installations and C:\Users for Vista users. Check to make sure that you've copied all of the files out of your profile before you upgrade. Be sure to also take a look at the hidden files and folders. These settings are necessary if you want to restore your old profile later, and you can always copy the data into your new installation, too.
That only takes care of what's in your profile, though. You can restore the full installation by using a backup program like Norton Ghost or one of the open source alternatives (Pack Rat Studios put together a good list) to create a full image of your system. If you want to go back, just use the image to restore the system. Follow the directions to create the image, and test the image to make sure the data is stored correctly before you install Windows 7.
Finally, it's best to do a clean install rather than an upgrade to your existing installation. That keeps old settings or data from hindering your upgrade.
This is an adventure, so be sure you are ready before you embark on it.