If you want to up grade to Windows 7 (to benefit from continued support), rather than paying Microsoft a fortune for the operating system – consider purchasing something like this:-
http://www.ebay.co.uk/itm/WINDOWS-7-32-BIT-PRO-DVD-WITH-GENUINE-COA-LICENSE-KEY-SCRAP-PC-/262960044728?epid=530532552&hash=item3d39a3dab8:g:YvoAAOSw42dZBbDn#rwid
I have purchased such a disk and used it to install on a PC purchased with no operating system.
If you do decide to buy such a disk, make sure it is suitable for your PC – most important 32 bit or 64 bit (the one above is for a 32 bit PC).
If you search ebay you will see than many sellers are selling what appears to be the same as above – but it is for a PC that already has Windows 7 installed and is only the COA license key. Check the ebay seller's listing carefully before buying.