If a deal for software seems too good to be true, then it probably is. The software could be pirated, which means that it's been copied and sold illegally.
One of the dangers of using pirated software is that you might not get security updates, which means that your computer and your personal information could be at risk.
One way you can usually identify genuine Microsoft software is by ensuring it has the Certificate of Authenticity (COA) label.
A typical COA looks like this:
For more information, see Protect your computer from pirated software.