The -12505 and -12514, "listener does not know of <sid>" or "...does not know of <service name>" those errors from an incorrect client setup. What <sid> or <service name> is the target for the connection attempts? At the command line a tnsping <alias> provides lots of information, e.g.
... used parameter files: ... <windows drive:>\<OH>\network\admin\sqlnet.ora
... OR (*nix) /OH/network/sqlnet.ora
... used TNSNAMES ...
... attempting to contact <connect string>
You're obviously talking to a listener, on the host specified, at the port specified. The listener is trying to tell you it can't connect you to the <sid> or <service name> you're asking for. What is your <connect string> from the tnsping output? That has the info. If its correct, or not, we can't tell either way. Its your instance.
For an ora-1034, the database instance you're trying to connect to, locally, is not running. Same for the -27101.
Message was edited by: clcarter fixed the darn slashes for win|*nix
Not a likely trouble cause, although Microsoft would much prefer one to use SQLServer instead of a competitor RDBMS
The XE install is intended to be a "one host, one database instance" setup, where some Oracle software installs used to include an "oracle home switcher" utility so that one could do a point and click to get the client set to use the desired ORACLE_HOME, which would be all the bits in the full path up to, but not including the network\admin folder names for the client setup folder.
The client setup in Windows depends on registry entries.
If the XE installer did not complete all its tasks successfully, that is one cause for the ora-1034 error- the problem is that the database is not running, and the ora-1034 error is a symptom of that problem. Usually about the only way to fix a "bad" install is follow the deinstall steps and run the installer again.
For Windows 7, which is newer than the XE 126.96.36.199 installer program, running the program via a right click/run as Administrator might get it to run correctly. Also note that no "home" releases of Windows are to be found on the list of "approved" host OSes, much better results are seen on the Professional version(s) of Windows.