For this being a Game Jam game, it's really nothing short of incredible. Or maybe a sign I need to stop doing Game Jams solo. Either way, this is a really neat concept. Pixel art is so overdone, but the actual theme of a Windows 95/98 computer looks fantastic and really gives a sense of charm to the whole game. The puzzle platformer thing is also pretty done, especially when it comes to indie titles, but it really does fit the theme.
Level design is tight and doesn't pull any punches. However, I found a lot of the earlier levels to actually be harder, whereas later levels were longer but easier. Most of them felt pretty well-balanced, though the final level was a bit too long (the difficulty was appropriate, it felt like the puzzle elements should have been spread among two levels instead of one.) Fortunately the one level where I felt the difficulty was bordering on too much also happened to be the final level, so I also appreciate that the outlier also happens to be the closest thing this game has to a final boss.
There are really only two flaws to this game that I can think of. First, a few of the background elements are a bit too vibrant, making it seem as though the player could actually interact with them. A simple transparent black filter over them might make it a bit more clear what's part of the level and what isn't. Second, seeing the virus tap the keys is useful, but it's really annoying that you have to look at the other end of the screen to see what's going on. The amount of looking back and forth between the controls and the game world can feel frustrating in a way that doesn't feel entirely fair. There are a handful of things that could have been done to make this a bit easier to digest, such as having some kind of wind-up animation to show when the virus was about to hit a key, but I can also see these making the game too easy at some points as well. Overall I think there's a way you could have telegraphed the key presses without sacrificing too much of the game's central mechanic.