VBA check if file exists

A way that is clean and short:

Public Function IsFile(s)
    IsFile = CreateObject("Scripting.FileSystemObject").FileExists(s)
End Function

something like this

best to use a workbook variable to provide further control (if needed) of the opened workbook

updated to test that file name was an actual workbook - which also makes the initial check redundant, other than to message the user than the Textbox is blank

Dim strFile As String
Dim WB As Workbook
strFile = Trim(TextBox1.Value)
Dim DirFile As String
If Len(strFile) = 0 Then Exit Sub

DirFile = "C:\Documents and Settings\Administrator\Desktop\" & strFile
If Len(Dir(DirFile)) = 0 Then
  MsgBox "File does not exist"
Else
 On Error Resume Next
 Set WB = Workbooks.Open(DirFile)
 On Error GoTo 0
 If WB Is Nothing Then MsgBox DirFile & " is invalid", vbCritical
End If

I use this function to check for file existence:

Function IsFile(ByVal fName As String) As Boolean
'Returns TRUE if the provided name points to an existing file.
'Returns FALSE if not existing, or if it's a folder
    On Error Resume Next
    IsFile = ((GetAttr(fName) And vbDirectory) <> vbDirectory)
End Function

For checking existence one can also use (works for both, files and folders):

Not Dir(DirFile, vbDirectory) = vbNullString

_{The result is True if a file or a directory exists.}

Example:

If Not Dir("C:\Temp\test.xlsx", vbDirectory) = vbNullString Then
    MsgBox "exists"
Else
    MsgBox "does not exist"
End If