No, don't tell them about dual booting. They may write you off. Just say a virus hit the machine.
When you get your key be sure to write it down along with your other stuff.
-> Is vista still running? Why not run BELARC ADVISOR (the free version) and print out that report with your keys?
To be as brief as possible:
I had been dual booting Vista and Ubuntu on my laptop. I decided to get rid of Ubuntu (and install it on another computer) but I was forced to install Vista again. No problem with that, only I have been asked to provide a product key for Vista and the sticker underneath my laptop that does provide that number has completely faded.
Now I understand I have to call the computer manufacturer and/or Microsoft, but if I explain how I've been dual booting will this pose a problem for me?