For starters you need to get this out of the Form_Load event
CODE
Dim StrInput As String
StrInput = InputBox("Enter Computers Username")
The only place StrInput will be visible is in the Form_Load as thats where it was created. This needs to be global so the whole code can see and use it. Once you've done that, can create a string variable like so
CODE
Dim whatToKill = "C:\Documents and Settings\" & StrInput & "\Local Settings\Temporary Internet Files\*.*"
Then do your normal work from there
CODE
Private Sub Temp_Int_Click()
Dim whatToKill = "C:\Documents and Settings\" & StrInput & "\Local Settings\Temporary Internet Files\*.*"
Kill whatToKill
End Sub
You also might want to put a message box somewhere in there to ensure the
whatToKill is getting the value you're hoping for
CODE
Private Sub Temp_Int_Click()
Dim whatToKill = "C:\Documents and Settings\" & StrInput & "\Local Settings\Temporary Internet Files\*.*"
MsgBox(whatToKill)
Kill whatToKill
End Sub
Hope this helps

Happy Coding!