Yacsui.dll is a DLL file that belongs to Yahoo! Audio Conferencing. If you have this program on your computer, and you have received an error related to this file then you most likely have an error in your registry. This is not anything to be alarmed about. It happens with some regularity, but it needs to be corrected in order for your computer to be running at optimum speed and performance.
You can use a trusted registry cleaner such as ZookaWare PC Cleaner to clear the errors in your registry. ZookaWare PC Cleaner even offers a free scan that will be able to tell you whether or not you have additional errors on your computer. Having smooth running registries is vital to the proper functioning of your computer. ZookaWare PC Cleaner has a 100% error removal guarantee. Most errors are cleared on the first scan or within 24 hours.