I used to host a little freeware app that I wrote in NS Basic called "GlucoCalc", which simply converted between serum glucose and HgbA1C values. Then I lost the source code and couldn't update it, so I took it down. I still get requests for it from time to time.
Now, you can download Diabetes Converter, a new freeware app from PDA Consult that does pretty much the same thing that GlucoCalc did, except that it looks a lot nicer. ;-)