Application is hanging when database upgraded from SQL Server 2000 to SQL Server 2008 R2

Presently we are working with SQL Server 2000, the database is perfectly working with our software for retail sales counter application developed with VB 6.0. But after upgraded to SQL Server 2008 R2, we are witnessing timeouts. We didn't change code and database structure when we upgraded from 2000 to 2008 R2.

This is the Locking Function for getting max number from table LockFile

 Private Function LockDatabase()
    Dim cntr As Long
    Dim indx As Integer
    Dim rec_Lock As New ADODB.Recordset

    'On Error GoTo CheckReTry

ReTry:
    'Set rec_Lock = db_kshetr.OpenRecordset("select LockSemaphore, BillNo as bno  from LockFile", dbOpenDynaset, , 2)
    rec_Lock.Open "Select LockSemaphore, BillNo as bno  from LockFile", con, adOpenDynamic, adLockPessimistic
TryGetLock:
    If rec_Lock.Fields("LockSemaphore") <> gCounter & "Locked" Then
        While (rec_Lock.Fields("LockSemaphore") <> "UnLocked")
          '  For indx = 0 To 10 + Rnd(25)
          '  Next indx
            rec_Lock.Requery
        Wend

        'rec_Lock.Edit

        If rec_Lock.Fields("LockSemaphore") = "UnLocked" Then
            rec_Lock.Fields("LockSemaphore") = gCounter & "Locked"
            rec_Lock.Update
            rec_Lock.Requery
        Else
            rec_Lock.CancelUpdate
            GoTo TryGetLock
        End If
    End If

    While (rec_Lock.Fields("LockSemaphore") <> gCounter & "Locked")
        For indx = 0 To 10 + Rnd(25)
        Next indx
        rec_Lock.Requery
    Wend

    counter = Val(rec_Lock.Fields("bno") & "")
    Set rec_Lock = Nothing
    Exit Function
CheckReTry:
    MsgBox "Cannot Lock Database", vbCritical + vbOKOnly, "Security"
    Exit Function
End Function

This is the UnLocking Function for getting max number from table LockFile

Private Function UnlockDatabase()

    Dim cntr As Long
    Dim indx As Integer
    Dim rec_Lock As New ADODB.Recordset

    On Error GoTo CheckReTry

ReTry:
    'Set rec_Lock = db_kshetr.OpenRecordset("select LockSemaphore, BillNo as bno  from LockFile", dbOpenDynaset, , 2)
    rec_Lock.Open "Select LockSemaphore, BillNo as bno  from LockFile", con, adOpenDynamic, adLockPessimistic
TryGetLock:
    While (rec_Lock.Fields("LockSemaphore") <> gCounter & "Locked")
        For indx = 0 To 10 + Rnd(25)
        Next indx
        rec_Lock.Requery
    Wend
    'rec_Lock.Edit
    rec_Lock.Fields("LockSemaphore") = "UnLocked"
    rec_Lock.Update
    rec_Lock.Requery

    While (rec_Lock.Fields("LockSemaphore") <> "UnLocked")
        For indx = 0 To 10 + Rnd(25)
        Next indx
        rec_Lock.Requery
    Wend
    Set rec_Lock = Nothing
    Exit Function
CheckReTry:
    MsgBox "Cannot UnLock Database", vbCritical + vbOKOnly, "Security"
    Exit Function
End Function

LockSemaphore Field Stages are Locked and UnLocked

Because we're having multiple clients, if one user locked the Lockfile table and get the maximum BillNo , an another user tries to get maximum BillNo from the same table at the same time, the whole systems will hang including Server(SQL Server 2008 R2).

How can we solve this problem? Thanks in advance.