I used to be able to play Skyrim with heavily modded graphics, though lately the FPS drop has been extreme. Same for Morrowind with MGSO. Now I found out that I get low FPS in unmodded FO:NV, even when the graphics are set at medium. So there's definitely something fishy going on.

My hypotheses have been the following:

A background program running - doesn't seem to be the case.

A driver issue - currently testing this out.

GPU being too hot - it is. The fan seems to be working. There was a lot of dust in my computer, and I've cleaned it - still low fps, and the GPU is still too hot. I hope I don't need to buy a new graphics card.

Anyway, I looked inside my computer again today, and found that there's a very fine layer of dust on the graphics card that I can't remove with the vacuum cleaner (I've been very careful with the vacuum cleaner, holding it a little above the surface). This could be what's causing it. Any idea how I can remove this dust layer without damaging the computer?

In Future please do not use a Vacuum cleaner to clean your computer system.

It is best to use a can of compressed air designed for cleaning computers.

The dust on the graphics card might be causing the extra heat and in return sluggish performance when playing games.

You can possibly try running the system with the case open depending on how your system is set up. See if there is improvement with the temperatures on the graphics card. What is the temperature on the graphics card. A good portion of cards can take quite some heat before they start to throttle down the performance. Does your graphics card have a fan.

You generally speaking do not want to run the system with the case open, but it can be useful to see if your cooling system on the PC is working correctly.