Are you allowing your phone to install 'non-Market' .apk files?
Menu > Settings > Applications > check Unknown Sources
If you haven't already, it's highly recommended to install an anti-malware app.
The .apk files you're installing may be corrupted or have become corrupted during download; this is more likely to happen if you're downloading them straight to your phone over mobile data, rather than WiFi. You can try downloading from a PC, then transfer the .apk to your phone.
If it's an ASTRO problem, then uninstalling it and downloading it from Android Market would be the best way to ensure you're getting the latest working version.