Eugene's observations

There is a strong likelyhood that at some stage of your Windows computer's life it will benefit from having software reinstalled.  Getting the installation kit for most programs is fairly straight forward, locating the product key or serial number that enables the product to be activated is often challenging.

This would be a good time to make sure you have a note somewhere of the product keys.  For many computers I can often glean this from Windows itself *if it is still functioning properly* but quite often the keys (that are held within the registry) get corrupted - perhaps at the time of an abrupt power down or by malware activity. 

At times like this, for Windows, you can sometimes look on the back of the computer at the Certificate of Authenticity which contains a key.  If your CoA sticker has become worn or defaced it may not be readable - hence, always make a note somewhere else.

Further, for some off-the-shelf mass produced computers the key on the CoA is a generic one that can not be used to reinstall Windows.  For such systems having the 'restore' CDs that came with or were available for you to create when the system was new will be essential.