If the board has 3 - 4 AGP/PCI/ISA slots, it's probably MicroATX. If it has 6+ slots, it's standard ATX.
There will also be a chance that it has proprietary design. Check the back of the computer and see if it has ATX I/O configuration.
Another thing which you need to take it into consideration is the power supply. The power supply in the case may not give enough juice to run the newer processors like P4 and Athlon. Especially, most P4 motherboards require 4 pin 12V connection, which your power supply most definately does not have.
This guy is asking for a similar question, and you can find some useful information in some of my responses although your system is a lot older than his.