If you plan on doing anything that might use DirectX, it can't hurt to update it every once in a while
Download the latest version from here:
DirectX 9.0C (Aug 2009)
Once it's downloaded, just run the executable, which will extract to a folder of your choice, then run the 'Setup.exe' file inside that folder.
As for finding out the DirectX version you are running, bring up a 'Run' command (WinKey + R), and type dxdiag.
It should give you a version number in there, but as you have XP on your netbook, it's most definitely going to be DirectX 9.0