What is the best way to buy windows 7 product key cheap or free?

Buying a Windows 7 product key cheaply or for free carries significant dangers. Sites offering cheap or free keys are often fronts for distributing malware.  The key itself might be legitimate, but the software or processes used to obtain or activate it could infect your system.  This can lead to data loss, identity theft, and control of your computer by malicious actors. Even if the key is legitimate, it might be linked to a previous user or be from a pirated copy of Windows 7.  Microsoft has robust systems to detect and prevent this, and your attempt to activate the key might trigger a security alert and even have your account suspended.

You won't have legitimate support from Microsoft if you use a pirated copy.  If something goes wrong with Windows 7, you're on your own without official assistance. Using a pirated or counterfeit Windows 7 product key is illegal in most jurisdictions. You could face fines or other penalties if caught. Malicious software accompanying a fake key could encrypt or delete your data.  This is especially dangerous for personal files and sensitive information. Malicious software or a pirated copy could degrade your system performance and potentially damage your hardware. 

Microsoft no longer supports Windows 7.  Even if the free Windows 7 product key is valid, you won't receive security updates.  This leaves your system extremely vulnerable to exploits.  The software accompanying the key may be poorly written and unreliable, causing further complications.