Buying used PC's, Security warning.
You can find some really good deal on eBay for used PC's and if you need one for school or your business this could be a cost effective to do it. I bought a used HP 8000 Elite for $110, great little machine and small, perfect for a project I am working. One of the reasons I bought this PC is it had Windows 7 installed and the COA sticker, Windows 7 Pro can cost $100 so this is like getting the OS with a free PC.
Now for the Security warning part, the seller of the PC I bought said Windows 7 Pro was installed and it was, so I decided look into this OS and check it. I wanted to make sure the installed OS was safe and not modified and legitimate. The first thing I did was to find the CD key in the installed OS and see if it matched the COA, the CD key in windows 7 is encrypted so you need a third part application to display, I used Magical jellybean key finder and the key that this OS had was not the one on the COA, this leads me to believe that this is not a legitimate OS and may be modified and cannot be trusted.
The easy solution was to download Windows 7 Pro from HeiDoc.net, these are legitimate versions of Windows 7 via Microsoft Azure, look at your COA and download the same version, if your COA says Windows 7 Home your CD key will not work if you download Windows 7 Pro.
If you buy a used computer, do not automatically trust the OS that installed on it, it may be modified or even hacked and you don't want use an OS that could be compromised and lose personal information.