FileMaker 8 was released quite a bit before windows 7. I've seen enough other reports here to suggest that this may be a compatibility issue between windows 7 (or Vista) and this older version of FileMaker. (most report this with FMP 7, however, so I could be wrong about version 8)
If so, then your only solution may be to upgrade FileMaker to a newer release.
Thank you for your post.
PhilModjunk is correct (thank you!), versions earlier than FileMaker Pro 10 have not been tested or certified with Windows 7. It is strongly recommended you use current FileMaker software with the Windows 7 operating system.
FileMaker Pro 7 through FileMaker Pro 11 do share the same file format. You will be able to open your FileMaker Pro 8 files with FileMaker Pro 11. I would recommend you make sure you have good backups of your solution whenever you plan any major changes.
You can find a trial version of FileMaker Pro 11 at www.filemaker.com. Go to the Support menu and select Downloads; you'll find a link to Trial Software at the bottom of the page under "Downloadable Resources".
Please note FileMaker recommends that, once you have created or opened a file in a later version of FileMaker Pro, you do not make database design or layout changes using an earlier version of FileMaker Pro, particularly for features that have changed in the newer version. You can also open FileMaker Pro 11 files using FileMaker Pro 8. However, any new features that are used in a later version of a FileMaker Pro file may not look or act the same way when you open the file in a previous version.
To purchase FileMaker Pro 11, see the "Buy One, Gift One" promotion running through December 15, 2011 (www.filemaker.com: pull down the Purchase menu and select Promotions).