my laptop recently had a replacement hard drive installed now theirs a watermark telling me to activate windows since windows was pre-installed i don't have a activation key. is it possible to retrieve or get a replacement key or do i have to buy a new copy of windows     

Have you tried to Activate it, does it ask for new product key?

You may contact your PC manufacturer's support for this issue.

Hi,

Yes, hardware changing might cause this kind of issues, if you are using retail version of Windows and the original key cannot activate Windows, try to activate it by phone.

https://support.microsoft.com/en-us/kb/950929

if your laptop was using OEM license, since it is completely hosted by OEM manufacturer, try contact with machine manufacturer for further information as mentioned by Cyber_Defend_Team.

Regards,

D. Wu