Declare Sub Sleep Lib "kernel32"... vb6

Your declaration is correct...something else is wrong.  If you put the declaration in a Module then change the Private to Public.

Here is working code with the exact same declaration as you had:

' -------------------------------------
' Form1
' -------------------------------------
Option Explicit

Private Declare Sub Sleep Lib "kernel32" (ByVal dwMilliseconds As Long)

Private quit As Boolean

Private Sub Form_Load()
    quit = False
    Me.Show
    Call mainloop
End Sub

Private Sub mainloop()
    Dim endTime As Date
   
    endTime = DateAdd("n", 5, Now()) ' 5 minutes from now...
    Do While Not quit
        If Now < endTime Then
            DoEvents ' keep your app responsive
            Sleep 50 ' keep CPU cycles down
        Else
            endTime = DateAdd("n", 5, Now()) ' another 5 minutes from now...
           
            ' do something in here...
           
           
        End If
    Loop
End Sub

Private Sub Form_Unload(Cancel As Integer)
    quit = True
End Sub

Your declare is correct for VB6:

Private Declare Sub Sleep Lib "kernel32" (ByVal dwMilliseconds As Long)

'Try it out
Sub Command1_Click()
    Call Sleep(1000)
End Sub

That is not caused by Sleep()....

Sleep() will cause your program to use absolutely NO CPU cycles.

100% CPU usage results from a tight loop such as this:

    Do While True
        ' do something without calling DoEvents and Sleep()
    Wend

The problem you describe isn't related to Sleep()...

Anyway, we are getting off topic.

Seems a bit strange though.  I mean, Kernal32.DLL is a pretty big component for windows to be missing.  I would think that other programs would be having trouble missing that DLL.  Maybe this is a system path issue.  Kernal32.DLL is located in your System32 directory.  I guess you can make sure it is located there (be sure to have explorer set up to show all files and not hide extensions when you look for stuff in your System32 directory).  If it is then maybe copy (that's COPY not MOVE) it over to the directory where your .EXE is running (just to test to see if it finds it in your .EXE directory).  If you run the program and Sleep works then maybe you have messed up your windows path or something.  Keep in mind that copying Kernal32.DLL over to your working directory is not the solution.  It is only a test to see if the Kernal32.DLL file is working.  Delete it from your working directory after you tried it out.

If you still get the error then it is likely that either Windows is corrupted or your VB installation is messed up.