We've seen a couple of issues in the last few weeks where customers are seeing a 1601 failure during the Visual Studio install.

The windows installer error code list shows this problem as:

   The Windows Installer service could not be accessed. Contact your support personnel to verify that the Windows Installer service is properly registered.

If this occurs, try the following to fix your machine:

   "c:\windows\system32\msiexec.exe /regserver"

If that doesn't work, you can try reviewing some of the common Windows Installer failures at:

http://support.microsoft.com/default.aspx?scid=kb;en-us;555175