Be aware that if you install Oracle 10 G Express on a PC already having a previous Oracle installation - then you'll have to merge tnsnames.ora files of the 2 installations or re-locate your Oracle HOME to the Oracle version that you want to use.
The TNS listener (Oracle stuff) will look in the first HOME dir only.
You might see the error message that TNS could not be found - in that case you probably can resolve it by merging tnsnames.ora files.