I love the do-Organizer program, but it is not compatible with 64-bit Windows 7? Alternativeto.com lists Mozilla Thunderbird as a similar program, is there a way to make Thunderbird function like do-Organizer?
If you give us a list of features that you want the alternative tool to have we can be more helpful.
Or you could use free online to-do applications like RememberTheMilk. The advantage of such services is that you can access your list from anywhere - including your cell phone(s), as long as the devices can connect to the net.
Thunderbird is a nice email alternative to programs like Outlook and can be a good way to manage your contacts, but if you're looking for something a little more specific, why don't you try searching MakeUseOf? We have tons of free alternatives and open source programs that you might find helpful, and you might find something closer to do-organizer than Thunderbird.
There's a really nice add-on called Lightning, which will add a full calendar and task list to your Thunderbird... I don't know whether it'd be enough to substitute do-Organizer, but it's worth a try for sure.