I am actually very keen, constantly upgrading my skills and I love tackling stuff that's a really formidable challenge, the problem is getting enthusiastic about XYZ Co.
Until recently I've been running my own 1-man software house and I think it's fair to say I prefer the service provider - client relationship over employee-employer any day. Problem is it was scarcely viable before and now in a nosedive economy it's a no-hoper.
Well, here's your answers.
When they ask why you want the geek job with XYZ Co., you say you want the job because you're very enthusiastic about problem-solving, you like challenges, and you believe that the job is both within your capabilities, but sufficiently challenging so as to give you job satisfaction.
And then say something like you've had a lot of experience having a lot of responsibility and autonomy, and having to resolve problems, liaising with clients directly, and this means that you're quite good at delivering to a brief, thinking of the end user and their needs, whether that's an external client, or an internal client.
Blah blah blah.
Turn it around, when they ask you questions about why you want to work for company XYZ, make your answer less about 'Ooh, I absolutely love the idea of working for a company that manufactures and sells widgets' and make your answers more about the job itself and how you can be enthusiastic about the tasks you'll be required to do, as opposed to what the company's end product is.
Or something like that.