I have a non genuine widows xp and I am having problems with it I need advice on where to buy cheapest genuine software
Allan

Windows is probably one of the most commonly pirated pieces of software, so I would buy from a shop you can trust. I would be very wary of places such as eBay. Ebuyer are selling XP Home Edition OEM for £74.32 http://www.ebuyer.com/product/114048 This is the OEM edition, so can be installed on one PC.

Are you 100% sure its none genuine? Windows tells you its none genuine if you haven't validated it with Windows genuine advantage. Is there a sticker on the computer with a product key? If so, you probably have a licence and can get it validated by following the prompts and entering that product key. If you get stuck, take it to a professional, who should be able to sort it for you.

Download this little program http://www.giveawayoftheday.com/freeware/product/key-finder
It will find the products licence key.

I tend to use the "magic jellybean finder" - if there is no sticker.

If it is a dodgy product key (eg duplicated or obsolete volume licence key) then you will either need to buy a genuine licence - as Rich has linked, or consider Windows 7 - you will need to download the Windows 7 upgrade advisor http://www.microsoft.com/downloads/en/details.aspx?FamilyID=1b544e90-7659-4bd9-9e51-2497c146af15 to see if your machine can run it. Or you install Ubuntu or another Linux variant - which are totally free