I'm going to move your post to the off topic folder as it isn't OneCare related.
They may be able to tell you what the LiveID is if you associated it with a PayPal account already and began the process for obtaining a rebate for a purchase already.
However, I can't find a link for how to get help from the Live Cashback program directly, so you'll be best served by going at it via Windows Live ID.
Go to http://support.live.com and sign in with a LiveID
Click on the Windows Live service or product you need help with.
If you are not presented with an email support form, but are instead redirected to help.live.com for that service/product, in the page that opens from http://help.live.com select any topic on the left hand side or search for a question in the search box.
The results of your search or the clicked topic appears in the right hand frame, along with a tiny link in the lower right hand corner – “Get more help.”
Click that tiny link and you will get to a page with a link “Contact support”
Click that link and you will be presented with the email support form to fill out. Provide as much detail as possible and you should get a response from support within 24 hours.
Help topics at http://help.live.com
-steve
