Xp have extremely low DOS compatibility so it is very unlikely that an old game manage to run fine without external help.
The most common problem is sound. VDMSound is a nifty little program designed especially for that. Thus, VDMSound should be used if you can manage to run a game fine under XP but can't manage to make the sound work. Is is also a workaround for DOSBox because you need a moderatly high powerful computer to run DOSBox, which is the ultimate solution.
DOSBox emulate an old 486 completely. DOS, sound card, drivers, you mean it. That program also can be set to different game speed as you wish and thus can be used as a slowdown program for very old games. The only drawback is that it only emulate a 486. This mean that some new games (like C&C on this site) will have a very hard time to work even with a high end computer because altough the game can be run on a 486, a Pentium was recommended at the time. The other drawback, somewhat related, is that emulation require a lot of processing power. If you don't have a fast machine then you might end up not being able to play the game with DOSBox at "full speed" for the game.
Overall those two programs are extremely good. I don't see how we could play old games on XP without them or similar programs.
If you have further question or seek a little more info I beckon you to read
this thread first as it might answer a few questions.