|When you enter a character in a cell|
and before you press enter
can you see the character in the Formula Bar?
I still think it could be your Font Color.
Check to make sure there are no rogue Macros running
and also check for any Conditional Formatting.
Does it occur on all Sheets in the Workbook
or just one specific sheet?
What version of Excel are you using?
If your on 2007 or newer, try saving the workbook in a different format like .xls
and see if it continues to give the same problem.