Intenta esto, puedes reemplazar el character por su unicode.
void CleanForAll(string AppData, string folder)
{
AppData = AppData.Replace("ñ","\u00f1");
foreach (string sid in keys)
{
string appDataPath = Registry.GetValue(UsersPath.Replace("<SID>", sid), AppData, null) as string;
if (appDataPath != null) Clean(Path.Combine(appDataPath, folder));
}
}
void CleanForAll(string AppData, string folder)
{
AppData = AppData.Replace("ñ","\u00f1");
foreach (string sid in keys)
{
string appDataPath = Registry.GetValue(UsersPath.Replace("<SID>", sid), AppData, null) as string;
if (appDataPath != null) Clean(Path.Combine(appDataPath, folder));
}
}