I hope you're not using Mozilla, but using Firefox?
The latest version of Mozilla Application Suite was 1.7, and that was back in 2005.
The suite lives on as SeaMonkey, but that's at version 2.0.2.
And the latest Firefox is 3.6, so if you've tried to install a recent extension on a Firefox version as old as 1.0 or 1.5, it definitely won't work.
I'm not on Windows 7, but on Windows XP SP3, Firefox 3.6 can install Vimperator 2.2 (October 28, 2009) just fine.
I've usually only seen error -203 when a network problem (or a firewall) has got in the way and prevented Firefox from downloading the XPI.
It may be that you have an old version of Vimperator which has become corrupted somehow. Or it could be that your Firefox profile is corrupted.
To test with a clean profile, try shutting down all Firefox windows, then start Firefox from the command-line with:
firefox.exe -P some-new-profile
...and then try installing Vimperator into that profile, to see whether it's a problem with your current profile, or some more general issue.
The "Help > About Mozilla Firefox" menu option will tell you which version you have.
The latest is 3.6, and the build id (shown at the bottom of the About Mozilla Firefox dialog) says
Mozilla/5.0 (Windows; U; Windows NT
5.1; en-US; rv:1.9.2) Gecko/20100115 Firefox/3.6