Excel 2003 - cannot type in cells

I have an end user who was having problems typing into cells in Excel 2000. If she double clicks a cell the only thing that happens is she can select multiple cells and has the big plus sign cursor. The cursor is then stuck and you cannot release the selection, if you try to close the window with the red X, the cursor keeps selecting cells in the upper right hand corner of the doc and you cannot close the window. You have to use task manager to close the window after this happens.

I resolved this issue once by removing Excel 2000 and installing all of her Office apps from the same Office 2003 installation. This worked for about a month and now she is having the same issue with her new Excel 2003 installation.

MS Excel 2003, Windows XP Pro. It's not a permission issue or read only file. She should be able to make changes to it. It happens on all of her .xls files. File associations are fine. There's got to be an easier way then re-installing every time this happens. Has anyone else seen this? Why does this happen?