I think theres more than two. It depends on the version of windows it was compiled on/for and the compiler would've built in the dialogs for that version. Ive seen some even older ones for Windows 9x. Really Windows should override the dialogs for the latest version imo.
System Manufacturer/Model Number self built OS Windows 7 Professional 64-bit CPU Intel E8400 3GHz Motherboard Intel DX48BT2 Memory Kingston PC3-10700H 4Gb Graphics Card XFX Radeon HD 5850 BlackEd. Sound Card Asus Xonar DG Monitor(s) Displays 2x Samsung SM-T220HD 22" Screen Resolution 1680x1050 on two monitors
Keyboard Microsoft Comfort Curve USB Mouse Razer Diamondback 3G PSU Thermaltake ToughPower 850w Case Thermaltake Armor Cooling Scythe Mugen II Hard Drives OCZ Vertex 2 120gb 3.5" (OS)
Seagate Momentus XT 500gb
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2x Samsung F1 1Tb Internet Speed 8128/443
The top one is the "shell" file dialog and is "part of the OS" as it ships. Programs can call it if they want but they have no real control over it.
The second one is the "MFC" dialog which most programs use. It's part of the programming API, not the OS and it gets installed with the VC runtime libs. There are several permutations of it too, if you have enough software installed you may see 3-4 slightly different ones. But also, that dialog can be "extended" with custom controls so you may see even more permutations of that one over time.
So program written for the MFC one will remain in that frozen state as long as they live, while programs wrtten to launch the shell one will have the dialog change on them from S to OS and possibly the API as well.
For that reason and just general backwards compatibility it isn't really possible to just override the "old" one with the new one.
The main problem is that the system one has evolved faster than the MFC API one so the disparity is considerable at this point :/
System Manufacturer/Model Number Scratch built OS Windows 7 x64 Ultimate CPU i7 960 Motherboard Asus P6X58D Memory 12 Gig Corsair Dominator Graphics Card Nvidia 480 Sound Card Maudio Delta 44 + breakout box Monitor(s) Displays Dell UltraSharp U2410 24in and Samsung 21 dual monitors Screen Resolution 1920x1200 and 1280x1024
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Secondary: Segate baracuda 1.0 TB
HDs in AHCI mode. Internet Speed 15kbs down 4.5kbps up Other Info WEI 7.6
CPU & RAM 7.6
Hard disk 7.7