My excel seems to be handling cell referencing correctly. It is keeping the
old value and not updating the cell with new information.

Example

I type 100 into A1. I then type =A1 into say cell B10. At this point B10
shows 100. I then change cell A1, say by typing in 170. Cell B10 continues to
show 100. When I goto B10, press F2 then enter, it updates it to 170. If I go
back to A1 and then change it again to say 45, cell B10 now continues to show
170.

This is also happening at work. And also other people's version of Excel. It
is causing me massive headaches.

Could this be caused by a virus? Has anybody seen this?