Affordable Project Management Software

Honestly with the economy in the shape it is in now we do not have the money to spend on a bloated, big name piece of project management software. Currently we are looking at a trial of Ubuntu Project Management Software and while the program will be adequate for our needs, we would obviously like to do some further comparison. I understand that there are some open source projects out there and would like to know if any of the more full featured ones run on Windows 2000. We have pondered making a switch to Linux in the near future as well considering the issues with Vista but are not sure when this switch may take effect.