ACT data is stored in the Database folder unless you chose to store them in a different location. I'll give you instructions based on the default settings, but if you used a different location you'll need to backup the data from there.
The Database folder is located:
C:\Act\Database for ACT! 3.0
C:\Program Files\Symantec\Act\Database for ACT! 4.0
C:\My Documents\Act\Database for ACT! 2000
Each ACT database has 12 - 22 files depending on features you use. You should select all files in the Database that use the filename you assigned. Place them either in easy to understand folders on your desktop, or burn them to CD/DVD or put them on a thumdrive...
Next we will Uninstall and reinstall ACT.
Once backed up go to Start, Control Panel, Add/Remove program. Find ACT and uninstall it. Be sure to reboot when done, even if it doesn't ask you to.
At this point I would recomend doing some system cleanup. Get a registy cleaner, I use CCleaner as it's free but if you already have one feel free to use it. CCleaner is at www.ccleaner.com. Download, install, and run it...