Whilst Windows and Linux both have their own encryption technologies, I do not think there are any pre boot solutions that work on both.
Your best bet would be to use either a software based solution such as Truecrypt* or look at your usage habits and virtualise one in another... For example, if you do not do anything intensive in Linux, I would virtualise it so that you can use Windows as your primary OS and then use Bitlocker.
(*) I realise that this is not what you want, but there isn't really many encryption solutions that are cross platform, let alone pre boot.
EDIT -
Just read the documentation, and it looks like Truecrypt does/can do pre boot authentication... but I have never used this feature and cannot really give any advise other than to say Truecrypt is a good product!