If this is a theme you added then it may be in msconfig>Startup or msconfig>Services which the uninstaller sometimes cannot remove since it is running at the time and often will fail to prompt of this.
If possible System Restore to before you installed the theme. If this is not possible, try a Restore point before you uninstalled it, turn it off in msconfig>Startup and then msconfig>Services (after Hiding all MS Services), reboot, check again to make sure it's not in either place and the theme didn't start with Windows 7, then uninstall it again using Revo Uninstaller in Advanced Mode. Revo Uninstaller - How to To Completely Uninstall Programs
If you can't get back before you installed or uninstalled it and it is causing problems, try reinstalling it for the purpose of correctly uninstalling it. If this is not possible, try SFC -SCANNOW Command
to see if it has damaged System files.
I installed an Angry Birds theme for a friend's Christmas gift to her son and it worked fine but it did start up with Windows so that could be your problem here.