Is something that has to stay running in the background at all times?
I have an old slow WinME machine that has had a tooltip issue for many years, but not one of tooltips showing behind the taskbar. The issue I have had is tooltips failing to show at all, anywhere, in any app. They just suddenly stop working, then later start working again. I have never been able to figure out what is triggering it or how to fix it.
One of the apps I wrote even had a feature added to it specifically because of this problem. My clock application has the option to show the date in a dialog, for use when my tooltips stop working.
I wonder if this utility could help, but then again if it has to stay running in the background at all times, it wouldn't be too useful on that really low spec machine.