I've personally tested LastPass with Windows 7, and haven't run into the issue you describe. However, if LastPass turns out to be the culprit, let us know and we'll try to track it down.
Because all your encrypted data is stored on our servers, you can uninstall and reinstall LastPass without any data loss. To uninstall, simply go to:
Start -> Program Files -> LastPass -> Uninstall
When you're ready to reinstall, you can do so from:https://lastpass.com/lastpass.exe