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.
Here are some links for legal downloads of W7 iso images of install discs, I suggest you get copies of these for future use, you will still need a legal key to activate the software.
http://www.mydigitallife.info/2010/04/28/download-windows-7-iso-official-32-bit-and-64-bit-direct-download-links/
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If you have activation problems with legitimate keys, here are the toll free Microsoft activation hotline numbers for the USA
XP
1-888-571-2048
Vista
1-866-740-1256
Windows 7
1-888-725-1047