Looks like an MS file - another link to a source online
http://support.microsoft.com/kb/172991
for 32bit systems, put it in C:\windows\system32, and register it, by using the command window, change to the c:\windows\system folder, and enter the command:
Regsvr32 c:\windows\system\msmask32.ocx
for 64bit windows 7, put the ocx in c:\windows\syswow64
open the command window (run as administrator!), enter
Regsvr32 c:\windows\syswow64\msmask32.ocx