OK is it a full retail or OEM version. The license is different. However what you want to do violates all MS terms. Take a look here (http://techblissonline.com/windows-7-license-terms/.
MS is very clear that there be one user of the key only and that it is installed on one system only. Depending on the version (retail or oem) you may or may not re-use your key on another system if the original system is no longer in use. When you go to activate the key/system on your comp. it will be seen as a different one from your friends and will not activate. In short one key active on one system at a time (OEM versions may not be transferred to a second system even if the original is no longer in use). GL