All Pocket PCs use Windows CE as their underlying operating system. However, there are two different versions that run on the iPAQ 3635:
Pocket PC 2000 and
Pocket PC 2002. You can compare the screenshots of each (use the links above) to easily determine which you have.
If you have Pocket PC 2000, it's theoretically upgradable. However, it's not necessarily easy to find that upgrade nowadays...
--janak