Yup, I tried upgrading and then a fresh install to a different hard drive. Both were failures with problems similar to yours. Your computer is a bit faster than mine is (see HP sepcs below).
You definitely want more memory and a more powerful graphics card, but ultimately, I think you are looking at a new PC if you want to run Vista. I am guessing that your PC may also use an AGP video card and probably slow memory compared to what's used today.
My decision was to stop spending money on the old computer and use the money towards a new PC that can really handle Vista. For Vista, my baseline recommendation is a dual core PC with 2GB RAM, preferably a Core 2 Duo and a Nvidia 8500 video card. I went for a Quad core and do not regret it other than the extra electricity that it uses. But it never runs out of CPU!
But back to attempting to find a solution to your current problem, I would first look at the video card drivers. Get the latest drivers from the Nvidia website.
brittonv:
DanH_HP_m376n, I did not do an upgrade, I did a reformat install on this box. Vista runs fine on this computer, but it is not a fast machine.
It's a P4 3.4 Ghz, with 1 gig of memory, Nvidia 6600GT Video card. When recording in HD OTA the CPU Usage is between 7 - 25% with more then 50% memory available.
This is pretty much a dedicated Media Center "Server" in that nothing else is installed.
I have all the updates installed
1: Dimension 9200 - Vista Ult, 2.4GHz Q6600 Core2Quad, 4GB RAM, 160GB, 750GB, 500GB USB, Nvidia 8300GS, PVR-150, HDTV Wonder, Vbox 3560, X360, DMA2100
2: HP m376n - MCE 2005, 2.6 P4 w/HT, 1GB RAM, 250GB, 400GB USB HD, Nvidia 6200, Emuzed MAUI, X360