I was lucky enough to cobble together a 3 axis table using some dumpster finds from a printing company, and I have successfully run Kellyware(in Win98) to cut straight lines and square shapes. My problem is with circular interpolation, which I've heard is a problem in KCam if the X&Y leadscrews are (in my case)not the same pitch. Is there any known tweak for this? If not, does anyone know of a 3 axis program that can compensate for this that is either freeware or inexpensive?

Mach3 is free for up to 1000 lines of g-code, and only $159 if you want to remove that restriction. Only runs in Win2K or XP, though. www.artofcnc.ca (http://www.artofcnc.ca)

That was fast, thanks for the reply. I'm stuck with Win98 for now due to that OS being friendlier to my "sacrificial" PC, which is a PII 300Mhz with 256MB of RAM. I've been testing the rig with my old Stepper3 board, and plan to use one I got from Paul Brewington a while back once I get the rig calibrated to my liking.
About now I'm pretty miffed that today I can buy a 24X36 aluminum and steel constructed machine off Ebay for less money than I spent scrounging and adapting mismatched parts over the last five years.
The up-side is, I've learned a hell of a lot about how the mechanicals operate(and how to keep from killing them).

Gerry's right in that Mach3 is the way to go. If you have a slower PC TurboCNC does a good job of circles with different lead screws. Runs in DOS and I have it happily running on a P100 ...

I took a look at the TurboCNC page. I'll download their demo and hopefully one of the sample files will have a circle in it. Thanks to everyone for the leads.
When I get the time I will try to post some pics of my dumpster derived abomination for everyone's entertainment.

TCNC will do the job as you set the proper "inches/step" for each axis. I have not heard of anybody having a problem with mixed pitches on the axis.

If you want an other option - emc2 is an open source linux based machine control. I really like it and it is very powerful.
http://www.linuxcnc.org/

(I am not a linux person but it was well worth the learning experience)
http://www.electronicsam.com/images/KandT/Dapper.png
http://www.electronicsam.com/images/KandT/Lerman.JPG
http://www.electronicsam.com/images/KandT/axisubuntu.png

I remember reading on the DeskCNC board that when differing pitch screws are used the algorithm for creating circles hangs up the CPU for a moment longer (a problem magnified with lower speed CPU's) causing a delay in one axis that results in an ellipse being created instead of a circle. Apparently some software has a means to compensate for the difference.

I believe KCam will work for you. Have you matched the number of steps for each motor to give you 1" of travel for each lead screw? This is found in "Table Setup".

My mill uses 8000 for the X, 8000 for the Y, 4224 for the Z. It runs on Windows 98. I have never tried to cut G02 or G03 useing the Z axis though.
All three axises gives me 1" of travel when ask to.