it can be done............
but it's not a real simple thing.....

one way - you'd have to have a selection_change macro that targets
those specific cells, so when they exit the cell, the maco
automatically checks what's been entered & sees if it's "ok."  then it
would have to throw up a message box directing them back to the cell,
to fix it, and what's wrong with it.

an easier (but less foolproof) method would be to have a validation
message box with an input message that basically says what you said
below:

"this cell can only contain alpha or numeric characters, maximum of
10.
no spaces,-,&,%,#... etc.. in the fields"

but like i said, it won't PREVENT users from entering the wrong data.

another way - you probably could also do it with data validation & a
formula format, but i don't know how to do that, either.
try searching the newsgroup for "validation format"

susan

How about event code that strips out all but numerics or alphas as you enter
data?

Private Sub Worksheet_Change(ByVal Target As Range)
Dim rCell As Range
Dim i As Long
Dim sChar As String
Dim sTemp As String
   On Error GoTo ws_exit:
   Application.EnableEvents = False
   If Not Intersect(Target, Me.Range("A1:A10")) Is Nothing Then
           With Target
               For i = 1 To Len(.Text)
                   sChar = Mid(.Text, i, 1)
                   If sChar Like "[0-9a-zA-Z]" Then _
                           sTemp = sTemp & sChar
               Next i
               .Value = sTemp
           End With
   End If
ws_exit:
   Application.EnableEvents = True
End Sub

Gord Dibben  MS Excel MVP