Balloon Tips are well-meaning and you can always suppress the warnings by actually taking action on them. But with Windows 7's new Action Center, a unified destination for system notifications and warnings, you can get your fill of security and PC-maintenance heads-ups all in one place. The Action Center, which resides by default in the lower right corner of the taskbar, raises a little warning icon if something needs attention.
To disable these balloon tips, look on the taskbar for the Action Center icon (a little flag, possibly with a red X), right-click on it and select 'Open Action Center' then click on 'Change Action Center settings'. This will take you to the 'Turn Messages On and Off' screen.
Uncheck the various categories of advisory you'd rather not be warned of via Balloon Tips. Click 'OK', and Balloon Tips in the relevant categories should no longer appear. Just remember to check into the Action Center if you see a red X on the icon.