I need to read the pluses from an encoder and then have the computer put out 0-10 volts based on what spindle speed has been commanded. Can anyone tell me how I can accomplish this? Thank you for your time.

Check out the interface boxes by LabJack (www.labjack.com). They connect to a USB port and have AD, DA, and digital I/O. I have been using several of them for logging data here at work and they work great. They cost about $100.00 and are the best bang for the buck that I have found anywhere. You don't mention how fast you need to process data so I have no clue as to your exact requirements.
Robert

Check this out. This will do exactly what you want.
http://www.usdigital.com/products/etach2/