Vista - Compacting Windows Mail

Every time my box comes up to compact the windows mail after I exit out of it, i tell it OKAY to compact the files and folders, but it ALWAYS says it cannot compact the files because files are open or in use somewhere else - which is NOT the case. Craziest thing I ever saw! SO, is there another way to compact all the Windows Mail messages and folders??????? If so, please give me clear instructions to do this. Thank you so much.