Maybe you have tried this, but the first thing would be to restart your computer. There may be a resource running in the background that is interfering with the installation.
If a restart a retry doesn't work, the next most obvious thing to try is uninstalling ALL programs related to Office. This includes any other versions of Word, Excel, Access, Powerpoint, and probably Works or other related products you had previously. Then restart the computer and reinstall your Office trial. Make another post here if that still doesn't work, and I'm sure you will get some more tips.

