Hey Mary,
It seems that something is wrong with that plugin. (this is a joomla plugin and is not coded from me).
By the look of you error message there are 2 possibilities:
1. corrupt file upload
2. corrupt file...
So please download joomla 1.5.25 from joomla.org unzip it and get the plugins/system/mtupgrade folder and override it on your server.
Check if this solves the problem.
If it doesn't then I'll need ftp access to your site to debug it myself.
Kind regards,
Daniel
edit: look what I found:
forum.joomla.org/viewtopic.php?p=2329508
- it seems that your hosting is running php4. PHP4 is dead since a lot of time! Ask your hosting how to upgrade to php5. Neither joomla or hotspots will work properly on php4.