While there are a number of different vendors (including Microsoft) that
sell Windows XP through online retail outlets, it might be advisable to
use your local store for a purchase (that way if you have any problems,
the seller is close by).

Rick

Jody
The first step is to establish whether your PC can actually run XP - look
here, and follow the instructions
http://www.microsoft.com/windowsme/upgrade.asp

(Note that Laptops are much more likely to have problems in upgrading the OS
than desktops - check with your manufacturer whether they support XP on your
laptop, also!)