I was in Delhi last February and the Delhi Times was full of ads for weekend tours to Kathmandu. If you wanted something longer, I'm sure that would be available, too. On a quick Web search I found one that takes 2 weeks to get from Delhi to Kathmandu- includes a boat, trains and a bicycle rickshaw!

I do recommend a guided tour unless you're an extremely experienced traveler. In my one flight out of DEL (on my way back home), I found the place to be maddening- you wait in line to get into the airport (no admission without ticket for that day), then to check in, then to go through Passport Control and again for security procedures, which include pat-downs for everyone so it's very long. At any one of those places you could get delayed and miss your flight- I'd think an organized tour might have more clout to get everyone through, or at least they'd be responsible to find a solution if you were delayed. This is not just the point of view of a foreigner- one of the locals observed on this Board that airports in India are like bus stations but plains fly in and out of them! But, departure from Delhi gives you an opportunity to see Delhi, which I loved.

This is a trip I'd like to make, too, if I get sent to Delhi again, but I plan to wait till things calm down.