Disable Anti-Virus Software:
Disable your AntiVirus Software, and then test ACT!. If you are able to create a backup with the AntiVirus software disabled, consult your AntiVirus documentation for information regarding adding ACT! to the list of Exclusions.

Third-Party Software:
To ensure that a conflict with Third-Party software is not the cause of this issue, close all other applications (including AntiVirus software if possible) and test. If the issue remains with all other applications closed, restart your computer in Selective Startup mode and attempt to back up your database.

Please refer to the following Knowledge Base Answer for information on starting your computer in Selective Startup mode.

If you find that you are able to create a back up of your database while in Selective Startup mode, it is likely that a program running in the background is causing the issue.

If you click the Startup tab in the System Configuration Utility after restarting in Normal Startup mode, you will find a list of all programs that load upon startup. Try disabling a few of these at a time, and then reboot to use the process of elimination to determine the conflicting program.
Note: The System Configuration Utility is not available in Windows 2000.