As far as I know extended is only needed if you're a programmer and you develop .net applications. Still, I suggest the same as Stumped. Unless you have serious problems with lack of hard drive space, just leave it. It doesn't bite or hurt and you can be sure every .net application can find what it needs.
Of course if you want to get rid of it, you can. If anything stops working, you just reinstall it anytime.
CPU: AMD Phenom II X4 810 @ 3120MHz | RAM: Kingmax 2x2GB DDR2 800 @ 833MHz| MoBo: MSI K9A2 CF v1.0 (BIOS: 1.D)| GPU: Asus HD 6850 1024MB (DirectCu) @ 835/1135MHz | Display: L24FHD | PSU: PC Power & Cooling Silencer 750 Quad | OS: MS Windows 3.11 Pro x64