I'm not too sure about this. If you've sent the disk and key to a charity, they may well have either sold that on or kept it for themselves. Either way, its probably already installed on another PC now and therefore I would have thought that, if you tried to use the key,it would be illegal. FE may have a view?
bjh.: problem is, charity shops are hardly experts on Windows activation. I picked up a copy of XP Pro still in its box with CoA at a local charity shop for 50p. If it works, great - but I won't be heartbroken if it doesn't... (And anyone who pays more than that for any software from a charity shop needs their head examining... :-) )