I use this bit of code myself (your code added):
const string source = "D:\\22.rar";
string destinationFolder = source.Remove(source.LastIndexOf('.'));
System.Diagnostics.Process p = new System.Diagnostics.Process();
p.StartInfo.CreateNoWindow = true;
p.StartInfo.UseShellExecute = false;
p.StartInfo.FileName = "\"C:\\Program Files\\WinRAR\\winrar.exe\"";
p.StartInfo.Arguments = string.Format(@"x -s ""{0}"" *.* ""{1}\""", source, destinationFolder);
p.Start();
p.WaitForExit();
That will launch the WinRAR program in the background and return control to you once the file has completed un-rar-ing.
Hope that can help