It's quite likely that the fixing arm has become detached but that the heatsink is still stuck on with compound, and so is doing it's job of taking the heat away from the CPU. It could go on working like this for years. Most of heatsinks from the era when your computer was made located on lugs moulded into the CPU socket. You really have two choices. As the computer is working normally, you could just leave things as they are and hope the heatsink remains stuck for another 8/9 years, or you could buy another heatsink and fit it. If you do replace the heatsink, or remove and refit your old one, make sure that you remove EVERY trace of old compound before applying new.