I'm a noob, but...
OS = Win7
.Net 3.5 = Installed (included w/ Win7)
ModMii =v4.2.6
The invalid files are already being commented on. The custom.md5 helps.
The ModMii download freezing was happening to me until I did two things. 1) Put ModMii folder at the root of a drive like "C:\ModMii"...