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Try the manual deinstall steps in the 10g XE doc pages, "7.2 Manually Removing Oracle Database XE" at http://docs.oracle.com/cd/B25329_01/doc/install.102/b25143/toc.htm#CIHDDHJD
Specifically, the win installer cleanup step. Per the XE license you can't have more than one XE installation, if there are some pieces left over it can/will cause trouble with the installer.