According to the Eve Online forums, the latest patch to the game - the Trinity patch - is disabling Windows on some machines. Not to get too technical, the problem appears to be a duplication of an essential boot-file called boot.ini, used by both Eve's setup and Windows, and its deletion at the end of the install.
We anticipate that the Eve makers will resolve this issue asap with another patch, but in the meantime, go here and here for solutions. Worst case scenario: you'll have to re-install Windows or do a system restore.
