I used to get the same error myself periodically (Event ID: 13...Event Source: nv)...normally when either my OC wasn't stable, or the AGP bus was way out of spec (up around 100MHz on previously run VIA boards, depending on the divider setup)...especially on the nForce boards I've owned (the 8RDA+ didn't like the AGP bus speed at anything higher than about 72MHz, and the NF7-S doesn't like it when the bus is pushed higher than around 67-68).
A few things that come to mind...
a) Make sure you're running the latest BIOS revision for your board, and that your OC is perfectly stable. Try setting all the AGP options in the BIOS to their least aggressive settings, and disable any 'shadowing' options.
b) If you have the option of editing the FastWrites option in the BIOS, disable it...having FW enabled shows very little gain if any in real world applications, and more times than not causes instability depending on the overall makeup of the system.
c) Unstable memory timings can also cause the
nv error...run Memtest v3.0 through all 7 tests a number of times to determine if maybe your timings are set too aggressively.
d) Like I mentioned above, excessive AGP bus frequency can also contribute to this error...if you're running a NF2 board, make sure it's as close to 66 as possible for the most stable system. If you have the option to increase the 'AGP Voltage', try setting it to the next highest value.
e) Updating your VC drivers may help matters, but I doubt it. Normally the
nv error is attributed to an unstable bus or OC (from my experience anyway). It wouldn't hurt to make sure you have the latest drivers for your card, and if you've recently updated drivers...it's possible there's still remnants of the old driver set on your rig.
f) Check to make sure that no other devices are sharing the IRQ with the VC. Depending on the system it may not make a difference, while on others...it can (definitely something to check).
g) If you're OC'ing your card, try lowering the core and memory clocks a little at a time...the VC could be overheating, and causing the error (a very good possibility).
h) If you're running a VIA board, make sure you're running the latest version of the 4in1's...an NF2 board, the latest Unified driver set...Intel, the latest INF.