I keep Dell install disc for XP with sp3 handy, it will install on any OEM PC without using a key, it will not activate however, when it fails to activate you will be able to change the key, punch in the key on the COA sticker that is on the PC, it should activate now. This is not an easy way, you still have to find all the drivers, and customers will complain all their OEM pre-installed software is missing.
It is the responsibility of the owner to keep up with discs and keys, not yours.
On many PCs like Dell, HP, Acer, there is a recovery partition on the hard drive that can reload the OS clean. Go to the manufacturers website for instructions.
Link for legal downloads of Windows 7 ISO images of install discs: http://www.mydigitallife.info/2010/04/28/download-windows-7-iso-official-32-bit-and-64-bit-direct-download-links/ (I suggest you get copies of these for future use, you will still need a legal key to activate the software.)
For Windows Vista, here is a link to a Dell community web page that describes the process to download and recreate Windows Vista (with SP1) bootable DVDs.
If you have activation problems with legitimate keys, here are the toll free Microsoft activation hotline numbers for the USA:
- Windows XP: 1(888)571-2048
- Windows Vista: 1(866)740-1256
- Windows 7: 1(888)725-1047