Define “convenient” here – I’m confused!
If the product key that came with the XP installation media is invalid, it’s because it’s not a valid key – one or more of the alphanumerics in the key code you have is incorrect.
I suggest that you return the copy of the installation media to the vendor and ask for a new one with a new key. The only other alternatives – such as obtaining another key from somewhere else – entail actions that may be considered illegal where you live.
Depending on the model of the EEE, you may find other OSes useful – UNE is great for netbooks provided you don’t have a first generation SSD installed. Alternatively, you could look at Linux distributions employing LXDE or XFCE as their desktop environments. They’re both excellent options for low-spec laptops/netbooks with slow SSDs, particularly if you install it on a non-journalling filesystem such as EXT2.