To reproduce:
-add RadGridView and enable excel-like filtering;
-apply Windows7 theme
When clicking on the filter button, the filter-dialog that shows is not really wide enough, it cuts the Cancel-button. 

Workaround:
this.radGridView1.FilterPopupInitialized += radGridView1_FilterPopupInitialized;

private void radGridView1_FilterPopupInitialized(object sender,
    Telerik.WinControls.UI.FilterPopupInitializedEventArgs e)
{
    e.FilterPopup.PopupOpening -= FilterPopup_PopupOpening;
    e.FilterPopup.PopupOpening += FilterPopup_PopupOpening;
}

private void FilterPopup_PopupOpening(object sender, CancelEventArgs args)
{
    RadListFilterPopup popup = sender as RadListFilterPopup;
    Padding currentPadding = popup.ButtonsMenuItem.Padding;
    popup.ButtonsMenuItem.Padding = new Padding(currentPadding.Left,
        currentPadding.Top, 20, currentPadding.Bottom);
}