The latest upgrade of the Malicious Software Tool changed the name to Windows Defender and the new version has its own home page: Windows Defender (Beta 2) Homepage. You can download the Windows Defender program there, and if you use Windows Automatic Update it should keep it up to date.
Users of the WMST beta versions and Windows Update are being fed the upgrade to Defender automatically. Again, it is not clear whether it's being given to all users of Auto Update.
A significant difference between the WMST and Defender is that WMST merely scanned regularly. With Defender you can elect to have "real time protection." There are a few other differences outlined in the FAQ linked at the Defender Homepage, which I'd suggest you read if you're uncertain about whether you should install it.
I would recommend that "reasonably competent" WinXP/Win2K users may want to try it, but less experienced users may want to use alternative methods for now. IF IT FINDS malware, it may offer you "choices" that really inexperience users may not want to face up to. If you're willing to accept the "recommended action" without really knowing what's going to happen, even the least experienced should find it okay.