well i wanted to change an old laptop to an ssd and I was wondering whether I could in fact do this wothout the licence becoming invalid. it has a COA but it doesn't say oem or have a manufacturers name on it. I googled it and it said you could change most if not the motherboard; but i read conflicting opinions on whether you could change the main boot drive.
If you have a COA it's an OEM copy. If you bought the computer with it preinstalled it is an OEM copy.
Microsoft ties the OEM activiation to the hash signature of the hardware. It's a complex hash so it does allow some variance and won't necessarily be invalidated by changing a few components. I don't believe the exact mechanics are known but Microsoft's technical support has been known to reinitialize the activation if you ask nicely and send them proof of ownership. I changed the HDD on my old laptop once and had no issues so the only way to know for sure is to try it
I've never had any trouble changing it on my PCs; your mileage may vary though.
Don't take this as fact, but I think for every hardware change (other than the motherboard) you lose 1 point - you only need to reactivate if you lose 3 points, and the points are restored by 1 a month. Be aware that I could be wrong though.
I've swapped/updated the boot drives on various rigs running XP/Win7 and never had an issue. You'll be fine. I don't think Microsoft gives a hoot about HDDs and other peripherals such as GPUs, etc... they're primarily interested in the motherboard.