I am trying to change my Microsoft word and install equation editor onto my computer. How ever when it is almost all installed a notice comes up telling me INSTALLATTION ERROR: FILE NOT FOUND A required installation file M4561403.CAB could not be found. and then they ask for the original installation source. I dont have it I got the computer with Microsoft office already on it. how to i get around this?

It should have come with Systems Disks by Law, unless you bought the computer used and the original user kept it. You will need to get your hands on a copy of office. If they did not send a disk they might have an ISO on a backup partition that you can create a disk from. Call the manufacture (i.e. Dell, HP, Sony, who ever) and get tech support. They will show you how to create a disk or how to order one. In the mean time, is your Office Working? If not try uninstalling the calculator.