Would it be possible to upscale virtualbox using sweetfx? I want to play an old win95 games but it looks pixelated

No. Reshade works by "pretending" to be the DirectX (or OpenGL) driver so it can overlay the effects into the rendered game, it doesn't actually render (or override the render) of the game, so it cannot display at at a higher resolution. Most older games that use VirtualBox likely do not have resolution settings, but they might follow whatever resolution VirtualBox is in. That means that you should try raising the resolution of VirtualBox, or if that isn't an option in VirtualBox games, then using something like NVIDIA Control Panel to downsample VirtualBox. However, if it looks pixelated, then it likely isn't actually pixelated. Many older (and even newer games) use pixel art, which has nothing to do with the rendering resolution but instead has to do with the artistic design choice to have it use pixel art. In this case, there is nothing you can really do since it is the art style of the game. You may be able to find some mods online that changes the graphics from pixel art to another art style, but that is a long shot since most games (especially older ones) do not support modding. Even if the game does support modding, you still likely won't find any mods to change the art style since 99% of games that have a pixel art graphics style use a 2D engine, so the only other real choice for the art style is hand drawn, and hand drawn is even harder to make then pixel art. You will probably not find modders creating totally new hand drawn images and animations just for a (free) mod.