Assuming you are on a PC...

You should use Control Panel - Add/Remove Programs to uninstall.

If you just want to get rid of the message then try the following;

Close Excel first and On the Windows Taskbar

1) Start-Run- "C:\Program Files\Microsoft Office\Office12\excel.exe"
/unregserver - OK

2) Start-Run- "C:\Program Files\Microsoft Office\Office11\excel.exe"
/regserver - OK

See the space between exe and /regserver

Note that quotes are required because of spaces in the path
"Saigon's Mom" wrote:

For some reason, although I use Excel 2003, Excel 2007 showed up on my
computer recently and won't go away. It opens, asks for a serial number, says
I can use it for a trial period and opens up a shortcut on my startup menu.
How do I get rid of it?