This morning while loosing myself between the blogger links i found a nice blog which i wish i could find it 2 months ago while i was facing problems with Oracle XE installation. Blogs owner Sergio Leunissen is Oracle ACE and employee of Oracle as a Director of Linux Engineering Team.
If you are interested about Oracle, Application Express, and Linux there are good stuff to read on his blog.
Oracle Database 10g Express Edition (XE) Installation Issues Roundup entry helped me on the causes and solutions of the errors about installing Oracle Database 10g Express Edition.
you can find the solutions of
ERROR: ORA-01041: internal error. hostdef extension doesn't exist
ERROR: ORA-12638: Credential retrieval failed
errors from this blog.
have a nice reading.