The Alexa subkey really can't be considered spyware in any real sense of the word.
It comes with Internet Explorer, where it's responsible for the 'related sites' feature on your Internet Explorer Tools menu.
If you have Ad-Aware remove it, the related sites entry disappears, but when you repair or upgrade IE, it's restored again.
It's completely harmless though, and it's up to you to decide whether to leave it, or to have it removed.
I've told Ad-Aware to ignore it, and I hope the next version will stop targeting it, so that I won't have to keep explaining...
Here's the SpyBot FAQ about it:
Why does Spybot-S&D not detect and remove the Alexa key for the What's related function of IE like other anti spyware utilities?
BTW, as of the latest update Patrick has decided to add detection for this item, mainly because he's tired of answering questions about it.... LOL :
"Yes, SPyBot now replaces the original related.htm file pointing to MS and from there to Alexa with the dummy that points to Google.
You may still remember that I wanted to implement a user-choice for the search engine there, but in the last months MS has closed the IE scripting even more so all my attempts have failed.
And of course there are two or three mails each day asking why other programs detect this registry key while Spybot-S&D does not.
So I thought a replacement with one other search engine is better than no replacement at all. "
I hope that answers your question.