It kind of sounds like you have an event macro running.

If you open excel in safe mode:
close excel
windows start button|Run
excel /safe

Does it happen?

If no, you'll want to talk to the developer to find out why he or she did this.

===
Another possibility--if you're always typing the same number, maybe Autocorrect
is kicking in. (but if you type in any number and this happens, it's probably
not autocorrect).

Tools|autocorrect options|autocorrect tab|look for those characters that are
getting corrected.

wrote:

Excel will not allow me to change a numerical figure in a specific cell.
I've tried deleting it & typing over it, but when the cursor is moved to the
next cell the original figure magically appears again.

--

Dave Peterson