You might have to do this as administrator.
Load Windows Explorer or whatever the Win7 equivalent is - easiest way is to hold down the "Windows" key and press E
Find the MM9 folder - it may be in a folder named 3do
Find the saves (not the minisaves)
Find the savegame files you want, right-click on the filenames, click on "Properties" in the drop-down menu, and towards the bottom of the window that opens, you will find the word Attributes". Uncheck the box marked Read-only.