Sorry about the bad wrapping on the longer lines of code. Here is the code
reformatted to fit better...

Function BigDec2Hex(ByVal DecimalIn As Variant, _
Optional BitSize As Long = 93) As String
Dim X As Integer, PowerOfTwo As Variant, BinaryString As String
Const BinValues = "*0000*0001*0010*0011*0100*0101*0110*0111*" & _
"1000*1001*1010*1011*1100*1101*1110*1111*"
Const HexValues = "0123456789ABCDEF"
DecimalIn = Int(CDec(DecimalIn))
If DecimalIn < 0 Then
If BitSize 0 Then
PowerOfTwo = 1
For X = 1 To BitSize
PowerOfTwo = 2 * CDec(PowerOfTwo)
Next
End If
DecimalIn = PowerOfTwo + DecimalIn
If DecimalIn < 0 Then
BigDec2Hex = CVErr(xlErrValue)
Exit Function
End If
End If
Do While DecimalIn < 0
BinaryString = Trim$(Str$(DecimalIn - 2 * _
Int(DecimalIn / 2))) & BinaryString
DecimalIn = Int(DecimalIn / 2)
Loop
BinaryString = String$((4 - Len(BinaryString) Mod 4) _
Mod 4, "0") & BinaryString
For X = 1 To Len(BinaryString) - 3 Step 4
BigDec2Hex = BigDec2Hex & Mid$(HexValues, (4 + InStr(BinValues, _
"*" & Mid$(BinaryString, X, 4) & "*")) \ 5, 1)
Next
End Function

Rick Rothstein (MVP - Excel)