Nearly every business that uses computers finds spreadsheets invaluable. But there's inevitably consternation if more than one person is working on the same file. The puzzle is tracking multiple versions of a file and figuring out who changed what, when--and why. Users of Microsoft Excel 5.0 for Windows can now get some help from QuikTrak (Softool Corp., 805 683-5777). The $99 Excel add-in makes it easy to save multiple versions of a spreadsheet. It also maintains a log that tracks who made changes, along with an explanation of the reasons for an update. With a couple of mouse clicks, you can compare any two versions of a file. Altered cells are instantly highlighted on the screen (above, in red).