I also found this which seems to explain what Shinmila H couldn't, why this happens.
And yes, i tried this solution, and it worked like a charm.
Often times Windows will refuse to hide because it mistakenly believes there is still a tooltip balloon open. (I see this a lot myself with APC's Powerchute Personal Edition.) Unfortunately the only way to get your taskbar to properly hide again (and I don't
mean uncheck the "Keep the taskbar ontop of other widows" option for a quick fix) is to restart the machine so that Windows resets the tooltip balloon status. What a pain!
Thankfully through my inability to let it go and hatred for this problem I have written a file that will fix this! (Works great on my Vista machine!) All the program does is overwrite the phantom tooltip balloon and then clear the tooltip balloon and Tahdah!
The tooltip will stay up for 10 seconds so that you can read the info on it (just telling you what it does and how you can contact me if you have any problems). You can close this tooltip balloon like any other by clicking on it, or if you mouse over the program
tray icon it will close itself instantly.
http://www.multiupload.com/94SQZ50YK0
I really hope this helped.
I've been fighting this for about a week now, since booting up the "spare," a newer PC than my work computer (which went to HP for repairs). Finally found this thread and tried nicolaj82's fix for the taskbar. Like he said, worked like a charm!! Taskbar
Slapper is the name of the program. lol
Since Microsoft won't get off their butts to fix this... after TWO YEARS... you might try installing this very small program from its Rapidshare location. You'll be working properly within just a few minutes!
Thanks to nicolaj82 for coming up with an answer after MS failed. Again.
Just keep bumping this and we'll all be in good shape... without "assistance" from Microsoft.