It might also be something to do with administrative rights on your 'server'.
Could also possibly be due to a change in your user profile if you've recently changed your user domain or are not using the same log on as when you installed XE.
SOme applications give one the option during installation to make the application available for 'this user only' or 'all users'. Can't remember if Oracle XE does this.
I had XE running quite nicely on my work lapotp until the end of last week when our IT administrator changed my laptop to the company domain. Now I get the same error but haven't had time to investigate fully yet.
The same error (system 5) occurs if I try to stop XE running.