What do you mean you removed the sticker? Is this a branded PC (not the one in your specs) which came with Windows 7 preinstalled? Because if so you must have the Product Key on the COA sticker attached to machine, or run Factory Recovery from its partition (if still there) or disks you made or order from Tech Support.
If it's a retail copy then you can audit the key as Shawn suggests using Belarc Advisor
however that key will not work once you're reinstalled or if you still have the factory Preinstall which has not been reinstalled yet and activated with the COA sticker's Product Key. The key used at factory to activate is a batch SLP key which cannot be used to activate a Clean Reinstall.
If you clarify a bit more then we can sum up your options a little better.