you can see if they have entered only valid characters by using InStr and Asc(i) in a loop.. something like this
this checks letters only, you could change this to include numbers or whatever, i just cant remember which numbers should be included, you could also check this against lowercase characters or whatever
strTextToBeChecked = "hello $ how are you today jy13?"
blnIllegalChars = False
For i = Chr(A) to Chr(Z)
If InStr(UCase(strTextToBeChecked), Asc(i)) Then blnIllegalChars = True
If blnIllegalChars = True Then
Response.Write "illegal chars entered!"