firstly im a noob when it comes to computers...
next.
I have a genuine product key sticker for xp stuck to my pc.

I fitted a new harddrive and the store installed xp back onto my harddrive

When i go to "my computer" and check "properties" it says that XP is OEM edition

My product key will not work when trying to activate it

I assumed my key was OEM tho it doesn't say so on the sticker.. should it say it?

How can i tell whether my key is retail or OEM?

Any ideas how to figure this out?

actually if u have an OEM version of windows it will say it on the sticker, and if u try to use that key on a version of windows that is not OEM it will not work and vicey versa.

You need to reinstall your own copy of XP.