1. Click on the start button and navigate to All Programs -> Accessories.
2. Right click on Snipping Tool, left click on Properties.
3. Click on the Shortcut tab.
4. Click your mouse in the 'Shortcut Key' field.
5. Think of a key combination (that is not already used by windows for something else) that you would like to use when you want to take a screenshot, then hold them down.

Example: Mine is Ctrl + Shift + `

6. The 'Shortcut Key' field should now reflect your key combination.
7. Click apply, then ok.
8. Now when you press this key combination, you can grab a screenshot easily.