If you look at "Computer" on your menu you should see the memory card as a "drive" with data on it. Right click on it and choose "Eject". You might get a warning "Card is in use", but can still click "continue anyway" or something similar. You should see a pop-up that says "removeable media can be safely removed" or somthing similar.
Always do that when removing a card from the computer. (I always simply use drag and drop to transfer data from my cards, then I'm already in "Computer" to do the eject once all data is moved or copied.) It's the same as unplugging any USB device.
I agree with Dwain.... if you format the card in your computer, be sure to format it again in your camera before starting to shoot with it. In fact, it's a good habit to get in, always formatting in camera any time you put a card in... All data that was on the card prior will be lost, though. So be sure you have transfered all the images you want to keep.
If using Windows 7 64 bit and shooting RAW, you might not be able to see the images in Windows Explorer. There is an applet or codec you can buy for little money that will make the images viewable. I have it installed but can't recall the name. If using 32 bit operating system, Canon's codec should work...it's free, but there isn't a 64 bit version yet.