When you installed Oracle on a Windows machine (previous to version 10g), it would also install the Oracle Home Selector:
This way you could switch between different Oracle versions installed on the same machine. It would set the environment appropriately (including the modification of the PATH variable).
With the release of Oracle 10g (and above) the Home Selector has been moved into the Oracle Universal Installer, but it has become a quite "hidden" feature.
Oracle Express Edition (XE) doesn't come with a Home Selector at all.
There is a freeware tool called "Oracle Locator Express" from DbMotiveTools, which does an excellent job here.
Once you download and double-click on it, you will find an additional icon in your taskbar.
Right-click on it and it will show you all Oracle_Homes it can find on this machine. On my machine it recognized all homes ranging from version 8.0 up until 10g.
It certainly found XE and the XE client as well.