Could it be the bug(?), where you need to deselct 'Enable Missing Glyph Protection'.

Go to Preferences > Type and un-tick the Enable Missing Glyph Protection.

In my (clean) installation, it's not present in the Fonts folder, nor in my registry (at important places), and it doesn't show up in any software as well.

Hmm, it is different here. Using Windows7 Professional with CS5. The image is a copy of the fonts folder  I've made after clean install of Windows 7. I found Marlett there but I have put a description in the name of the copied folder that tells 72 fonts were visible in the Fonts folder even thought I have set Windows to show hidden files.  But after I've made the copy of it, a number of hidden fonts (with the dimmed icons) showed making the total of 196 fonts. Marlett was one of the hidden files. And it does show currently on the font menu in Illustrator. I can't remove it from the menu with a font manager and I can't delete it form the Font folder because it doesn't show there. However I don't have the OP's problem, when navigating the font menus with the arrow keys - I can select the next font without a problem. My problem with this and may other system fonts is that they clutter the fonts menu in Illustrator and there is no way to remove them. That's why I made several times wishes here that it would be great if Illustrator has some Font menu organization features that allow only selected groups of fonts to show.

Those instructions are for Windows 95 but basically they say to reinstall the system which is also the only way in Windows 7.

Not sure how reputable it is but this link has some information about the role of Marlett in Windows 7
http://www.prepressure.com/fonts/basics/windows-7-font-handling

a quote from it

Fonts that should never be deleted in Windows 7

I have not yet found a list of the mandatory system fonts. At the very least the following fonts should not be deleted:

• Marlette – This is the font used for the minimize, maximize and close buttons on your window borders.
• Arial (TrueType)
• Times New Roman (TrueType)
• Courier New (TrueType)
• Any .FON files. These are fixed size fonts used for MS-DOS windows and some dialog boxes.

 

If I enable the Glyph protection I will have the same problem with this and many other fonts, it should be always off and is a bad idea that they decided to be on by default.