Right-click on the sheet tab
Choose "View Code"
Within the menu click Insert \ Module
Paste in the code below
Close the VBA editor
Press Alt-F8
Choose the macro
Click Run
If that doesn't work check your security settings as described here:
Enable or disable macros in Office files - Office Support
Note: The macro removes any formulas in column C, because it is only possible to format a part of a cell if it contains a value.
Andreas.
Sub BoldName()
Dim Where As Range, This As Range
Dim i As Long
Set Where = Range("C1", Range("C" & Rows.Count).End(xlUp))
With Where
.Value = .Value
.Font.Bold = False
End With
For Each This In Where
i = InStr(This, ",")
If i = 0 Then i = Len(This) + 1
This.Characters(1, i - 1).Font.Bold = True
Next
End Sub