I am running Studio 12 Ultimate under XP Pro SP3 on a new Q9550 / 4gb RAM / 500gb SATA / nVidia 8600GT rig (and what a difference the quad processor with 12mb cache and SSE 4 makes in Studio!). I had the same situation - Imported from DV and created 2 hr video with a 4 chapter menu having motion thumbnails. It hangs at "Writing final file". I rebooted, rendered again with the same result.
Here's what I did:
1) Changed my swap file setting (Start > Control Panel > System > Advanced Tab > Performance section, Settings button > Advanced tab, Virtual memory section) to System Managed
2) Downloaded and installed the Studio 12.1 patch
3) Downloaded and installed the newest nVidia driver for my 8600GT
Rebooted and restarted Studio. Rendering worked successfully. I'm crossing my fingers that the hanging problem is resolved.
Studio 12 is a great upgrade from my old Studio 9.4, and the new hardware makes using the program with large video projects much more bearable. I highly recommend stretching for a Quad with SSE4 - your time is worth it!