Microsoft Malicious Software Removal Tool is an application to scan your PC and detect problems of infection because of specific malware (for example, viruses like Blaster, Sasser, MyDoom ... well-known). It also helps you eliminate these infections, if it detects them, from your system.
It becomes very useful to always run Microsoft Malicious Software Removal Tool, even when the PC is apparently suffering no problems. On many occasions, viruses take their time to manifest themselves while the PC acts normally. The idea is to use this program as an antivirus or utility in general that offers you permanent and up to date protection. Microsoft Malicious Software Removal Tool, repeated again, is for specific infections and viruses.