Install 1.0, quit firefox, replace your zotero data directory with the backup you made prior to installing 1.5, and then start up firefox. If you did not create a backup, you would have to export your data from 1.5, start 1.0 with an empty data directory, and import your data. This will break documents that you've written using the OO.o and/or Word plugins (item IDs change). You may lose other data through the export/import as well.
Do you have any complaints with 1.5 other than the aesthetic notes issues that you've raised in other threads?
Upgrade Storage
Do you have any complaints with 1.5 other than the aesthetic notes issues that you've raised in other threads?