The game might crash but Windows probably won’t. (Note the word “probably”)
Back in the day, there were two basic versions of Windows, the original version which was backwards compatible to DOS, and NT. These two operating systems looked very similar, but were fundamentally different in the way that they were structured. NT has what it calls the “hardware abstraction layer” which completely prevents programs from directly accessing hardware. Original Windows does not have this structure, and instead allows programs to directly access hardware if they want to.
The original Windows is version 1.x, 2.x, 3.x, Windows 95, Windows 98, and Windows ME. If you look at how Windows identified itself the versions become a lot clearer. Windows 95 is Windows 4.0, 98 is 4.1, and ME is 4.9.
NT versions are Windows NT 3.1, NT 4.0, Windows 2000 (NT 5.0), Windows XP (NT 5.1), Vista (NT 6.0), Windows 7 (NT 6.1), Windows 8 (NT 6.2), Windows 8.1 (NT 6.3), Windows 10 (NT 10.0).
Again, looking at the version numbers makes it a lot more clear about exactly what is going on. Windows 7, 8, and 8.1 weren’t really major changes to the operating system under the hood. While the user interface looked a lot different, the actual guts of the operating system did not change that much. With Windows 10, people were starting to pay more attention to the version numbers so Microsoft got rid of useful version numbers and let the marketing folks decide version numbers as well as OS names.
Home software, especially games, often accessed the hardware directly. Sometimes this was for backwards compatibility so that you could continue to use your old DOS programs. Sometimes it was for performance. Games would often bypass the operating system completely and would directly control the video card, for example. If they didn’t, then the games would not have run fast enough on the technology that was available back then.
Business applications often ran in networked environments with multiple tasks running, and in this environment, the HAL really proved its worth. So there was this natural divide where Windows was for home users and NT was for business users.
Microsoft got a little tired of maintaining two different Windows lines (original and NT), so they decided to “merge” the operating systems. This was pure marketing BS. They didn’t merge anything. What they wanted to do was kill of the original Windows line and force everyone to NT. Marketing BS aside, this was actually a good thing, as the HAL does tremendous things to prevent misbehaving programs from trashing your system or trashing other programs. So it’s actually a good thing that we’re all running NT at this point.
The big problem was that a lot of software, especially games, was written to directly access hardware so that the games could run fast enough on the available technology. And since the HAL didn’t allow this, those games would not run under NT.
Microsoft first tried their “merge” with Windows 2000. You could see evidence of this with some of the cutesy icon name changes between NT 4 and 2000 (My Network Neighborhood, for example). About halfway through development, Microsoft realized that this “merge” was going to fail miserably. There was just too much common home software out there that would not run under NT. But Microsoft did announce their intent to kill off Windows and force everyone to NT, and all of the software developers took notice. They put Windows 2000 back on the “business” track and abandoned plans to market it for home users, and rushed Windows ME out the door so that they would have something to sell to home users (which is why ME was such a buggy piece of crap when it first launched).
Microsoft successfully completed their “merge” (aka killing off Windows completely without actually merging anything) with Windows XP. What is important to note for this discussion is that games that wouldn’t run under Windows 2000 usually wouldn’t run under XP either. The only thing in Microsoft’s favor was that all of the newer software was designed to run on NT, and the games and programs that didn’t run under NT were now older and not as widely used.
So, what does all of this mean for OS and software compatibility?
You get into the biggest trouble when you jump OS lines, meaning going from any of the original Windows operating systems (95, 98, ME) to any version of NT (2000, XP, Vista, 7, etc). Your next biggest trouble comes from jumping major versions (i.e. XP to 7 since this is a change from Windows 5.x to 6.x). Jumping minor versions (Windows 7 to 8.0 for example) can break things but it’s not as likely.
98 to XP is a jump across Windows lines (Windows to NT) and is also a major version jump (4.x to 5.x), so depending on how the game was written, it might not run at all. The game also might not even install.
As far as what’s the worst that could happen, the worst that I have personally seen is that the game locks up the computer’s user interface, forcing you to completely reboot the computer. Theoretically I suppose it’s possible that the program could damage your installation of Windows, but I personally have never seen it happen and also have never even heard of it happening to anyone that I knew.
There’s no real risk of hardware damage. The HAL in XP should prevent even the worst misbehaving program from even accessing the hardware, let alone getting it into a state where it could theoretically damage something.
If this ancient XP computer still has an old fashioned CRT type monitor, you can damage the monitor by having the game switch into a resolution mode that the CRT wasn’t designed for. It won’t blow the monitor right away, but the extra strain on the CRT could cause it to die an early death. Modern monitors are immune to this.
If it were my system, I’d just try it and see what happens. Half the time the program will run anyway even though it wasn’t designed for NT. Even if the program doesn’t work, the worst that I would expect to happen is that the computer locks up and you have to hit the reset button. It’s much more likely that the program just won’t install or run.